UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 8-K

CURRENT REPORT
Pursuant to Section 13 OR 15(d) of The Securities Exchange Act of 1934

Date of Report (Date of earliest event reported) December 27, 2004

STRUCTURED ASSET SECURITIES CORPORATION (as Depositor under the Trust Agreement dated as of November 1, 2004, providing for the issuance of Structured Asset Securities Corporation Mortgage Pass-Through Certificates, Series 2004-21XS)
(Exact name of registrant as specified in its charter)

Delaware
(State or other jurisdiction
of incorporation)

333-115858
(Commission
File Number)

74-2440850
(IRS Employer
Identification No.)

745 Seventh Avenue, 7th Floor, New York, New York
(Address of principal executive offices)

10019
(Zip Code)

Registrant's telephone number, including area code 212-526-7000

(Former name or former address, if changed since last report.)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

[ ] Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

[ ] Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

[ ] Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

[ ] Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Section 8 - Other Events

Item 8.01. Other Events.

This current report on Form 8-K relates to the monthly distribution reported to the holders of Structures Asset Securities Corporation, Mortgage Pass-Through Certificates, Series 2004-21XS, which was made on December 27, 2004.

Section 9 - Financial Statements and Exhibits

Item 9.01 Financial Statements and Exhibits.

99.1 Monthly distribution report pursuant to Section 5.02 of the Trust Agreement for the distribution on December 27, 2004.

(Exhibit No. Description)

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

LASALLE BANK NATIONAL ASSOCIATION, IN ITS CAPACITY AS TRUSTEE UNDER THE TRUST AGREEMENT ON BEHALF OF STRUCTURED ASSET SECURITIES CORPORATION, REGISTRANT

By: /s/ Russell Goldenberg
Russell Goldenberg
Group Senior Vice President

Date: December 29, 2004

Structured Asset Securities Corporation

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Reporting Package Table of Contents

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Administrator:

Amanda Hellyer 312.904.6299
amanda.hellyer@abnamro.com

Analyst:

Dinsmore Sohn 714.259.6258
dinsmore.sohn@abnamro.com

Statement Date:

135 S. LaSalle Street, Suite 1625
Chicago, IL 60603
USA

Statements to Certificateholders
Component Detail Report
Cash Reconciliation Summary
Bond Interest Reconciliation

Shortfall Summary Report

Other Related Information
Asset-Backed Facts ~ 15 Month Loan Status Summary Part I

Asset-Backed Facts ~ 15 Month Loan Status Summary Part II
Asset-Backed Facts ~ 15 Month Loan Payoff/Loss Summary
Realized Loss Detail

Page 2-3
Page 4
Page 5-7
Page 8

Page 9

Page 10
Page 11-13

Page 14-16
Page 17-19

Page 20-22

Substitution Detail History

Page 23

Page(s)

Information is available for this issue from the following sources

Issue Id:

Monthly Data File
Name:

SAS421XS

SAS421XS_200412_3.ZIP

Closing Date:

First Payment Date:

Rated Final Payment: Date:

30-Nov-04
27-Dec-04

26-Dec-39

Parties to The Transaction

Depositor: Structured Asset Securities Corporation/Structured Asset Securities Corporation

Underwriter: Lehman Brothers Inc.

Master Servicer: Aurora Loan Services, Inc.

Rating Agency: Moody's Investors Service, Inc./Standard & Poor's

800.246.5761

LaSalle Factor Line

LaSalle Web Site

Servicer Web Site

www.alservices.com

www.etrustee.net

Page 1 of 23

Class

CUSIP

Original

Opening

Principal

Negative

Closing

Interest

Pass-Through

Per $ 1000

Next Rate(3)

WAC:
WAMM:
Current Index:
Next Index:

Payment Date:
Prior Payment:
Next Payment:
Record Date:

6.426873%

2.208000%
2.417500%

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Structured Asset Securities Corporation

Mortgage Pass-Through Certificates

Series 2004-21XS

Face Value (1)

Balance

Payment

Adj. or Loss

Amortization

Balance

Payment (2)

Adjustment

Rate

340

REMIC 3 (Distribution)

Statement Date:

ABN AMRO Acct: 722176.2

1000.000000000

74.713812813

0.000000000

925.286187187

3.034722215

0.00

0.000000000

4.37000000%

0.000000000

86359BN31

1-A1

81,790,000.00

6,110,842.75

0.00

75,679,157.25

248,209.93

1000.000000000

0.000000000

1000.000000000

3.575000000

0.00

0.000000000

4.29000000%

0.000000000

86359BN49

1-A2

32,522,000.00

0.00

32,522,000.00

116,266.15

1000.000000000

0.000000000

1000.000000000

3.700000000

0.00

0.000000000

4.44000000%

0.000000000

86359BN56

1-A3

33,857,000.00

0.00

33,857,000.00

125,270.90

1000.000000000

0.000000000

1000.000000000

4.474999917

0.00

0.000000000

5.37000000%

0.000000000

86359BN64

1-A4

60,101,000.00

0.00

60,101,000.00

268,951.97

1000.000000000

0.000000000

1000.000000000

4.166666523

0.00

0.000000000

5.00000000%

0.000000000

86359BN72

1-A5

23,141,000.00

0.00

23,141,000.00

96,420.83

1000.000000000

127.859628833

0.000000000

872.140371167

1.986111111

0.00

0.000000000

2.86000000%

0.000000000

86359BN80

2-A1A

46,044,000.00

5,887,168.75

0.00

40,156,831.25

91,448.50

1000.000000000

127.859629000

0.000000000

872.140371000

1.768500000

2.417500000%

0.00

0.000000000

2.35800000%

0.000000000

86359BN98

2-A1B

30,000,000.00

3,835,788.87

0.00

26,164,211.13

53,055.00

1000.000000000

0.000000000

1000.000000000

2.991666698

0.00

0.000000000

3.59000000%

0.000000000

86359BP21

2-A2

53,567,000.00

0.00

53,567,000.00

160,254.61

1000.000000000

0.000000000

1000.000000000

3.600000000

0.00

0.000000000

4.32000000%

0.000000000

86359BP39

2-A3

79,462,000.00

0.00

79,462,000.00

286,063.20

1000.000000000

0.000000000

1000.000000000

4.083333457

0.00

0.000000000

4.90000000%

0.000000000

86359BP47

2-A4A

27,014,000.00

0.00

27,014,000.00

110,307.17

1000.000000000

0.000000000

1000.000000000

4.483333333

0.00

0.000000000

5.38000000%

0.000000000

86359BP54

2-A4B

1,140,000.00

0.00

1,140,000.00

5,111.00

1000.000000000

0.000000000

1000.000000000

4.475000132

0.00

0.000000000

5.37000000%

0.000000000

86359BP62

2-A5A

37,759,000.00

0.00

37,759,000.00

168,971.53

1000.000000000

0.000000000

1000.000000000

4.825000000

0.00

0.000000000

5.79000000%

0.000000000

86359BP70

2-A5B

1,594,000.00

0.00

1,594,000.00

7,691.05

1000.000000000

0.000000000

1000.000000000

3.950000000

0.00

0.000000000

4.74000000%

0.000000000

86359BP88

2-A6A

29,486,000.00

0.00

29,486,000.00

116,469.70

1000.000000000

0.000000000

1000.000000000

4.291670683

0.00

0.000000000

5.15000000%

0.000000000

86359BP96

2-A6B

1,245,000.00

0.00

1,245,000.00

5,343.13

1000.000000000

0.000000000

1000.000000000

4.466666352

0.00

0.000000000

5.36000000%

0.000000000

86359BQ20

1-M1

10,586,000.00

0.00

10,586,000.00

47,284.13

Page 2 of 23

Notes: (1) N denotes notional balance not included in total (2) Accrued Interest Plus/Minus Interest Adjustment Minus Deferred Interest equals Interest Payment (3) Estimated. * Denotes Controlling Class

Class

CUSIP

Original

Opening

Principal

Negative

Closing

Interest

Pass-Through

Per $ 1000

Next Rate(3)

WAC:
WAMM:
Current Index:
Next Index:

Payment Date:
Prior Payment:
Next Payment:
Record Date:

6.426873%

2.208000%
2.417500%

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Structured Asset Securities Corporation

Mortgage Pass-Through Certificates

Series 2004-21XS

Face Value (1)

Balance

Payment

Adj. or Loss

Amortization

Balance

Payment (2)

Adjustment

Rate

340

REMIC 3 (Distribution)

Statement Date:

ABN AMRO Acct: 722176.2

1000.000000000

0.000000000

1000.000000000

4.708332621

0.00

0.000000000

5.65000000%

0.000000000

86359BQ38

1-M2

2,339,000.00

0.00

2,339,000.00

11,012.79

1000.000000000

0.000000000

1000.000000000

4.641667003

0.00

0.000000000

5.57000000%

0.000000000

86359BQ46

2-M1

9,898,000.00

0.00

9,898,000.00

45,943.22

1000.000000000

0.000000000

1000.000000000

4.833335944

0.00

0.000000000

5.80000000%

0.000000000

86359BQ53

2-M2

1,277,000.00

0.00

1,277,000.00

6,172.17

1000.000000000

0.000000000

1000.000000000

4.852768533

0.00

0.000000000

5.82332186%

0.000000000

86359BQ61

2,644,000.00

0.00

2,644,000.00

12,830.72

0.000000000

0.00

7,731.95

0.000000000

N/A

0.000000000

9ABST244

100.00

0.00

100.00

7,731.95

0.000000000

0.00

)

(909,837.79

0.000000000

1.93080218%

0.000000000

9ABST245

565,467,224.63

0.00

550,543,262.05

0.00

0.000000000

0.00

0.000000000

N/A

0.000000000

9ABST246

0.00

Total P&I Payment

0.00

)

(902,105.84

565,466,100.00

17,824,610.02

Total

549,632,299.63

15,833,800.37

0.00

1,990,809.65

Page 3 of 23

Class

CUSIP

Original

Opening

Principal

Negative

Closing

Interest

Pass-Through

Per $ 0

Next Rate(3)

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Structured Asset Securities Corporation

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Face Value (1)

Balance

Payment

Adj. or Loss

Amortization

Balance

Payment (2)

Adjustment

Rate

Statement Date:

Component Detail

0.000000000

0.00

0.000000000

5.75000000%

0.000000000

M3(1)

1,846,000.00

0.00

1,846,000.00

8,845.42

0.000000000

0.00

0.000000000

5.99293608%

0.000000000

M3(2)

798,000.00

0.00

798,000.00

3,985.30

Page 4 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Statement Date:

Cash Reconciliation Summary

Current Scheduled Interest

Interest Summary

Less Deferred Interest

Plus Gross Advance Interest

Less PPIS Reducing Scheduled Int

605,670.19

0.00

)

(19,628.27

Less ASER Interest Adv Reduction

0.00

Less Other Adjustment

0.00

Total

2,904,380.35

Unscheduled Interest:
Prepayment Penalties

7,731.95

Yield Maintenance Penalties

0.00

Other Interest Proceeds

0.00

Total

7,731.95

Less Fees & Expenses Paid By/To Servicer

Less Fee Paid To Servicer
Less Fee Strips Paid by Servicer

Special Servicing Fees

Interest Due Serv on Advances
Recoup of Prior Advances
Misc. Fees & Expenses

Total Unscheduled Fees & Expenses

Total Interest Due Trust

Less Fees & Expenses Paid By/To Trust
Trustee Fee
Fee Strips
Misc. Fees
Interest Reserve Withholding
Plus Interest Reserve Deposit
Total

Total Interest Due Certs

Principal Summary

Scheduled Principal:
Current Scheduled Principal
Advanced Scheduled Principal
Scheduled Principal
Unscheduled Principal:
Curtailments
Prepayments in Full
Liquidation Proceeds
Repurchase Proceeds
Other Principal Proceeds
Total Unscheduled Principal
Remittance Principal

Remittance P&I Due Trust

Remittance P&I Due Certs

Pool Balance Summary

Beginning Pool
Scheduled Principal
Unscheduled Principal
Deferred Interest
Liquidations
Repurchases

Ending Pool

Balance

Count

Principal

Interest

Prior Outstanding

Plus Current Period

Less Recovered

Ending Outstanding

Servicing Fee Summary

Current Servicing Fees
Plus Fees Advanced for PPIS
Less Reduction for PPIS
Plus Delinquent Servicing Fees

Total Servicing Fees

Overcollateralization Summary

PPIS Summary

Gross PPIS
Reduced by PPIE
Reduced by Shortfalls in Fees
Reduced by Other Amounts
PPIS Reducing Scheduled Interest
PPIS Reducing Servicing Fee

PPIS Due Certificate

0.00

0.00
0.00

0.00

2,906,822.30

0.00
0.00
0.00
0.00

0.00

86,223.18

271,336.87
357,560.05

148,039.64

14,418,362.89

0.00
0.00
0.00

14,566,402.53
14,923,962.58

17,830,784.88

565,467,224.63

1,691

357,560.05

14,566,402.53

0.00
0.00

0.00

550,543,262.05

1,654

0.00

271,336.87

2,318,338.43

271,336.87

2,318,338.43

24,918.27

0.00

)

(19,628.27

104,479.53

109,769.53

19,628.27

0.00
0.00
0.00

19,628.27
19,628.27

0.00

Advance Summary (Advance Made by Servicer)

)

(5,290.00

2,906,822.30

Interest Not Advanced (

Current Period

)

0.00

2,318,338.43

Workout Fees
Liquidation Fees

0.00
0.00

Less Non Recovered

0.00

0.00
0.00

0.00

Overcollateralization Amount

Overcollateralization Deficiency

911,062.42

785,338.58

Other Principal Information

Principal Remittance Amount
Overcollateralization Release Amount

14,923,962.58

0.00

14,923,962.58

Principal Distribution Amount Amount

Page 5 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Statement Date:

Cash Reconciliation Summary Pool 1

Current Scheduled Interest

Interest Summary

Less Deferred Interest

Plus Gross Advance Interest

Less PPIS Reducing Scheduled Int

232,994.49

0.00

)

(7,222.75

Less ASER Interest Adv Reduction

0.00

Less Other Adjustment

0.00

Total

1,305,720.87

Unscheduled Interest:
Prepayment Penalties

7,731.95

Yield Maintenance Penalties

0.00

Other Interest Proceeds

0.00

Total

7,731.95

Less Fees & Expenses Paid By/To Servicer

Less Fee Paid To Servicer
Less Fee Strips Paid by Servicer

Special Servicing Fees

Interest Due Serv on Advances
Recoup of Prior Advances
Misc. Fees & Expenses

Total Unscheduled Fees & Expenses

Total Interest Due Trust

Less Fees & Expenses Paid By/To Trust
Trustee Fee
Fee Strips
Misc. Fees
Interest Reserve Withholding
Plus Interest Reserve Deposit
Total

Total Interest Due Certs

Principal Summary

Remittance P&I Due Trust

Remittance P&I Due Certs

Pool Balance Summary

Beginning Pool
Scheduled Principal
Unscheduled Principal
Deferred Interest
Liquidations
Repurchases

Ending Pool

Balance

Count

Principal

Interest

Prior Outstanding

Plus Current Period

Less Recovered

Ending Outstanding

Servicing Fee Summary

Current Servicing Fees
Plus Fees Advanced for PPIS
Less Reduction for PPIS
Plus Delinquent Servicing Fees

Total Servicing Fees

Overcollateralization Summary

PPIS Summary

Gross PPIS
Reduced by PPIE
Reduced by Shortfalls in Fees
Reduced by Other Amounts
PPIS Reducing Scheduled Interest
PPIS Reducing Servicing Fee

PPIS Due Certificate

0.00

0.00
0.00

0.00

1,311,053.02

0.00
0.00
0.00
0.00

0.00

36,807.82

143,863.38
180,671.20

125,875.85

5,429,411.63

0.00
0.00
0.00

5,555,287.48
5,735,958.68

7,047,011.70

246,182,546.72

992

180,671.20

5,555,287.48

0.00
0.00

0.00

240,446,588.04

972

0.00

143,863.38

1,079,949.13

143,863.38

1,079,949.13

9,622.55

0.00

)

(7,222.75

51,416.90

53,816.70

7,222.75

0.00
0.00
0.00

7,222.75
7,222.75

0.00

Advance Summary (Advance Made by Servicer)

)

(2,399.80

1,311,053.02

Interest Not Advanced (

Current Period

)

0.00

1,079,949.13

Workout Fees
Liquidation Fees

0.00
0.00

Less Non Recovered

0.00

0.00
0.00

0.00

Overcollateralization Amount

Overcollateralization Deficiency

375,430.79

363,116.21

Other Principal Information

Principal Remittance Amount
Overcollateralization Release Amount

5,735,958.68

0.00

5,735,958.68

Principal Distribution Amount Amount

Page 6 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Statement Date:

Cash Reconciliation Summary Pool 2

Current Scheduled Interest

Interest Summary

Less Deferred Interest

Plus Gross Advance Interest

Less PPIS Reducing Scheduled Int

372,675.69

0.00

)

(12,405.52

Less ASER Interest Adv Reduction

0.00

Less Other Adjustment

0.00

Total

1,598,659.48

Unscheduled Interest:
Prepayment Penalties

0.00

Yield Maintenance Penalties

0.00

Other Interest Proceeds

0.00

Total

0.00

Less Fees & Expenses Paid By/To Servicer

Less Fee Paid To Servicer
Less Fee Strips Paid by Servicer

Special Servicing Fees

Interest Due Serv on Advances
Recoup of Prior Advances
Misc. Fees & Expenses

Total Unscheduled Fees & Expenses

Total Interest Due Trust

Less Fees & Expenses Paid By/To Trust
Trustee Fee
Fee Strips
Misc. Fees
Interest Reserve Withholding
Plus Interest Reserve Deposit
Total

Total Interest Due Certs

Principal Summary

Remittance P&I Due Trust

Remittance P&I Due Certs

Pool Balance Summary

Beginning Pool
Scheduled Principal
Unscheduled Principal
Deferred Interest
Liquidations
Repurchases

Ending Pool

Balance

Count

Principal

Interest

Prior Outstanding

Plus Current Period

Less Recovered

Ending Outstanding

Servicing Fee Summary

Current Servicing Fees
Plus Fees Advanced for PPIS
Less Reduction for PPIS
Plus Delinquent Servicing Fees

Total Servicing Fees

Overcollateralization Summary

PPIS Summary

Gross PPIS
Reduced by PPIE
Reduced by Shortfalls in Fees
Reduced by Other Amounts
PPIS Reducing Scheduled Interest
PPIS Reducing Servicing Fee

PPIS Due Certificate

0.00

0.00
0.00

0.00

1,595,769.28

0.00
0.00
0.00
0.00

0.00

49,415.36

127,473.49
176,888.85

22,163.79

8,988,951.26

0.00
0.00
0.00

9,011,115.05
9,188,003.90

10,783,773.18

319,284,677.91

699

176,888.85

9,011,115.05

0.00
0.00

0.00

310,096,674.01

682

0.00

127,473.49

1,238,389.31

127,473.49

1,238,389.31

15,295.72

0.00

)

(12,405.52

53,062.63

55,952.83

12,405.52

0.00
0.00
0.00

12,405.52
12,405.52

0.00

Advance Summary (Advance Made by Servicer)

)

(2,890.20

1,595,769.28

Interest Not Advanced (

Current Period

)

0.00

1,238,389.31

Workout Fees
Liquidation Fees

0.00
0.00

Less Non Recovered

0.00

0.00
0.00

0.00

Overcollateralization Amount

Overcollateralization Deficiency

535,631.63

422,222.37

Other Principal Information

Principal Remittance Amount
Overcollateralization Release Amount

9,188,003.90

0.00

9,188,003.90

Principal Distribution Amount Amount

Page 7 of 23

Structured Asset Securities Corporation

Class

Accrued

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Bond Interest Reconciliation

Prior

Other

Additions

Interest

Losses

Current

Cumulative

Accrual

Risk Carry-

Fwd Amt

Interest

Payment

Amount

Interest

Certificate

Interest

Losses

Shortfall

Int. Carry-

Proceeds

Risk Carry-

Certificate

Agreement

Amt Deposited

From YM

Method Days

Basis

Remaining

Statement Date:

Distributable

Prior

Applied Realized

Remaining
Int. Carry-

forward Amt

Principal

Principal
Principal

Extra

Prepayments

Shortfalls

Net
Cap

Rate
Y/N

0.00

1-A1

248,209.93

0.00

248,209.93

30/360

0.00

1-A2

116,266.15

0.00

116,266.15

30/360

0.00

1-A3

125,270.90

0.00

125,270.90

30/360

0.00

1-A4

268,951.97

0.00

268,951.97

30/360

0.00

1-A5

96,420.83

0.00

96,420.83

30/360

0.00

2-A1A

91,448.50

0.00

91,448.50

30/360

0.00

2-A1B

53,055.00

0.00

53,055.00

Act/360

0.00

2-A2

160,254.61

0.00

160,254.61

30/360

0.00

2-A3

286,063.20

0.00

286,063.20

30/360

0.00

2-A4A

110,307.17

0.00

110,307.17

30/360

0.00

2-A4B

5,111.00

0.00

5,111.00

30/360

0.00

2-A5A

168,971.53

0.00

168,971.53

30/360

0.00

2-A5B

7,691.05

0.00

7,691.05

30/360

0.00

2-A6A

116,469.70

0.00

116,469.70

30/360

0.00

2-A6B

5,343.13

0.00

5,343.13

30/360

0.00

1-M1

47,284.13

0.00

47,284.13

30/360

0.00

1-M2

11,012.79

0.00

11,012.79

30/360

0.00

2-M1

45,943.22

0.00

45,943.22

30/360

0.00

2-M2

6,172.17

0.00

6,172.17

30/360

0.00

12,830.72

0.00

12,830.72

30/360

0.00

4.70

0.00

7,731.95

0.00

909,837.79

0.00

909,837.79

0.00

30/360

0.00

909,837.79

0.00

2,892,915.49

2,900,647.44

1,990,809.65

0.00

909,837.79

4.70

(2) Principal Prepayments include the Extra Principal Amount

Page 8 of 23

(1) Other Interest Proceeds are additional interest amounts specifically allocated to the bond(s) and used in determining the bonds Distributable Interest.

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Interest Adjustments Summary

Statement Date

0.00

Special Servicing Fees

Workout Fees

Liquidation Fees

Legal Fees

0.00

Misc. Fees & Expenses Paid by/to Servicer

Interest Paid to Servicer on Outstanding Advances

0.00

ASER Interest Advance Reduction

Interest Not Advanced (Current Period)

Recoup of Prior Advances by Servicer

0.00

Servicing Fees Paid Servicer on Loans Not Advanced

Misc. Fees & Expenses Paid by Trust

0.00

Shortfall Due to Rate Modification

Total Shortfall Allocated to the Bonds

Total Interest Adjustment to the Bonds

0.00

Shortfall Allocated to the Bonds:

Excess Allocated to the Bonds:

Other Interest Proceeds Due the Bonds

Yield Maintenance Penalties Due the Bonds

Prepayment Penalties Due the Bonds

Recovered ASER Interest Due the Bonds

Recovered Interest Due the Bonds

Less Total Shortfall Allocated to the Bonds

Total Excess Allocated to the Bonds

7,731.95

7,731.95

Total Excess Allocated to the Bonds

7,731.95

0.00

Aggregate Interest Adjustment Allocated to the Bonds

0.00

7,731.95

0.00

Prepayment Interest Excess Due the Bonds

0.00

ARD Excess Interest

0.00

Net Prepayment Int. Shortfalls Allocated to the Bonds

Other Interest Loss

Page 9 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Other Related Information

Statement Date:

$6,174.87

Insurance Premium...................................................................................................

Insured Payments to Class:

I-A2......................................................................................................................

I-A4......................................................................................................................

0.00

Page 10 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Asset-Backed Facts ~ 15 Month Historical Loan Status Summary Part I

Distribution

Date

REO

# Balance

Statement Date:

Foreclosure

Delinq 3+ Months

Delinq 2 Months

Delinq 1 Month

Current

# Balance

Delinquency Aging Categories

0.18%

0.14%

0.00%

27-Dec-04

752,462

1,651

549,790,800

99.82%

99.86%

Page 11 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Asset-Backed Facts ~ 15 Month Historical Loan Status Summary Part I

Distribution

Date

REO

# Balance

Statement Date:

Foreclosure

Delinq 3+ Months

Delinq 2 Months

Delinq 1 Month

Current

# Balance

Delinquency Aging Categories

Pool 1

0.10%

0.09%

0.00%

27-Dec-04

216,121

971

240,230,467

99.90%

99.91%

Page 12 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Asset-Backed Facts ~ 15 Month Historical Loan Status Summary Part I

Distribution

Date

REO

# Balance

Statement Date:

Foreclosure

Delinq 3+ Months

Delinq 2 Months

Delinq 1 Month

Current

# Balance

Delinquency Aging Categories

Pool 2

0.29%

0.17%

0.00%

27-Dec-04

536,341

680

309,560,333

99.71%

99.83%

Page 13 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Distribution

Date

Statement Date:

Current 31 - 60 Days 61 - 90 Days 90 + Days
# Balance # Balance # Balance # Balance

In Foreclosure and Delinquent

In Bankruptcy and Delinquent

In REO and Delinquent

Asset-Backed Facts ~ 15 Month Historical Loan Status Summary Part II

Current

31 - 60 Days

61 - 90 Days

90 + Days

Balance

27-Dec-04

0.00%

0.00% 0.00%

0.00%

Page 14 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Distribution

Date

Statement Date:

Current 31 - 60 Days 61 - 90 Days 90 + Days
# Balance # Balance # Balance # Balance

In Foreclosure and Delinquent

In Bankruptcy and Delinquent

In REO and Delinquent

Asset-Backed Facts ~ 15 Month Historical Loan Status Summary Part II - Pool 1

Current

31 - 60 Days

61 - 90 Days

90 + Days

Balance

27-Dec-04

0.00%

0.00% 0.00%

0.00%

Page 15 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Distribution

Date

Statement Date:

Current 31 - 60 Days 61 - 90 Days 90 + Days
# Balance # Balance # Balance # Balance

In Foreclosure and Delinquent

In Bankruptcy and Delinquent

In REO and Delinquent

Asset-Backed Facts ~ 15 Month Historical Loan Status Summary Part II - Pool 2

Current

31 - 60 Days

61 - 90 Days

90 + Days

Balance

27-Dec-04

0.00%

0.00% 0.00%

0.00%

Page 16 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Asset-Backed Facts ~ 15 Month Historical Payoff/Loss Summary

Distribution

Date

Ending Pool (1)

Payoffs(2)

Realized Losses (2)

Remaining Term

Curr Weighted Avg.

# Balance

# Amount

Life

Coupon Remit

Statement Date:

Ending Arrearage (1)

Balance

Insurance

Substitution

Liquidation

Proceeds

27-Dec-04

1,654

97.81%

550,543,262

97.36%

2.19%

2.55%

0.00%

340

6.43%

6.15%

14,418,363

0.00

(1) Percentage based on pool as of cutoff. (2) Percentage based on pool as of beginning of period.

Page 17 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Asset-Backed Facts ~ 15 Month Historical Payoff/Loss Summary

Distribution

Date

Ending Pool (1)

Payoffs(2)

Realized Losses (2)

Remaining Term

Curr Weighted Avg.

# Balance

# Amount

Life

Coupon Remit

Statement Date:

Ending Arrearage (1)

Balance

Insurance

Substitution

Liquidation

Proceeds

Pool 1

27-Dec-04

972

57.48%

240,446,588

42.52%

2.02%

2.21%

0.00%

327

6.65%

6.35%

5,429,412

0.00

(1) Percentage based on pool as of cutoff. (2) Percentage based on pool as of beginning of period.

Page 18 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Asset-Backed Facts ~ 15 Month Historical Payoff/Loss Summary

Distribution

Date

Ending Pool (1)

Payoffs(2)

Realized Losses (2)

Remaining Term

Curr Weighted Avg.

# Balance

# Amount

Life

Coupon Remit

Statement Date:

Ending Arrearage (1)

Balance

Insurance

Substitution

Liquidation

Proceeds

Pool 2

27-Dec-04

682

40.33%

310,096,674

54.84%

2.43%

2.82%

0.00%

350

6.25%

6.00%

8,988,951

0.00

(1) Percentage based on pool as of cutoff. (2) Percentage based on pool as of beginning of period.

Page 19 of 23

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Realized Loss Detail

Period

Disclosure

Control #

Appraisal

Date

Appraisal

Value

Beginning
Scheduled

Balance

Gross

Proceeds

Gross Proceeds

as a % of

Sched. Balance

Aggregate

Liquidation
Expenses *

Net

Liquidation

Proceeds

Net Proceeds

as a % of

Sched. Balance

Realized

Loss

Statement Date:

Current Total
Cumulative

Page 20 of 23

* Aggregate liquidation expenses also include outstanding P&I advances and unpaid servicing fees, unpaid trustee fees, etc..

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Realized Loss Detail

Period

Disclosure

Control #

Appraisal

Date

Appraisal

Value

Beginning
Scheduled

Balance

Gross

Proceeds

Gross Proceeds

as a % of

Sched. Balance

Aggregate

Liquidation
Expenses *

Net

Liquidation

Proceeds

Net Proceeds

as a % of

Sched. Balance

Realized

Loss

Statement Date:

Pool 1

Current Total
Cumulative

Page 21 of 23

* Aggregate liquidation expenses also include outstanding P&I advances and unpaid servicing fees, unpaid trustee fees, etc..

Structured Asset Securities Corporation

Payment Date:
Prior Payment:
Next Payment:
Record Date:

27-Dec-04
27-Dec-04

N/A

25-Jan-05

24-Dec-04

Mortgage Pass-Through Certificates

Series 2004-21XS

ABN AMRO Acct: 722176.2

Realized Loss Detail

Period

Disclosure

Control #

Appraisal

Date

Appraisal

Value

Beginning
Scheduled

Balance

Gross

Proceeds

Gross Proceeds

as a % of

Sched. Balance

Aggregate

Liquidation
Expenses *

Net

Liquidation

Proceeds

Net Proceeds

as a % of

Sched. Balance

Realized

Loss

Statement Date:

Pool 2

Current Total
Cumulative

Page 22 of 23

* Aggregate liquidation expenses also include outstanding P&I advances and unpaid servicing fees, unpaid trustee fees, etc..

Structured Asset Securities Corporation

Substitution Detail History

ABN AMRO Acct: 722176.2

Series 2004-21XS

Mortgage Pass-Through Certificates

24-Dec-04

25-Jan-05

N/A

27-Dec-04

Record Date:

Next Payment:

Prior Payment:

Payment Date:

Statement Date:

Property ID

Deleted Loan

Qualified Substitute

Scheduled Prin Bal

Page 23 of 23

Original Property

Per Section 7.01(a)(i) of the Property Management and Lease Servicing Agreement, aggregate Appraised Value of the Qualified Substitute Mortgaged Properties acquired by the Issuer since the Closing Date
in connection with the substitution or exchange pursuant to Section 7.01 is not to exceed 25% of the aggregate Initial Appraised Value of the Mortgaged Properties.

(A)

Structured Asset Securities Corp Mortgage Pass-Through Certificates, Series 2004-21XS 8-K