# United States

# SECURITIES AND
EXCHANGE COMMISSION

# Washington, D.C. 20549

# Form 10-K

ANNUAL REPORT PURSUANT TO
SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 FOR THE FISCAL YEAR ENDED
DECEMBER 31, 2002

WASHINGTON MUTUAL
MORTGAGE SECURITIES CORP. (Exact name of registrant as specified in its charter)

|Delaware                                       |  | 333-77026-01             |  | 94-2528990                          
|---------------------------------------------- |  | ------------------------ |  | ------------------------------------
|(State or other jurisdiction
of incorporation) |  | (Commission File Number) |  | (I.R.S. Employer
Identification No.)

1201 THIRD AVENUE, WMT
1706 SEATTLE, WASHINGTON 98101 (206) 377-8555 (Address, including zip code, and telephone number, including area code, of registrants
principal executive offices)

Securities registered pursuant to Section 12(b) of the Act: None
Securities registered pursuant to Section 12(g) of the Act: None

Indicate
by check mark whether the registrant (1) has filed all reports required to be filed by
Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months
(or for such shorter period that the registrant was required to file such reports), and
(2) has been subject to such filing requirements for the past 90 days. Yes [X] No [ ]

Indicate
by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is
not contained herein, and will not be contained, to the best of registrants
knowledge, in definitive proxy or information statements incorporated by reference in Part
III of this Form 10-K or any amendment to this Form 10-K. [ ] Not applicable.

State
the aggregate market value of the voting and non-voting common equity held by
non-affiliates of the registrant: None.

Indicate
the number of shares outstanding of each of the registrants classes of common stock,
as of the latest practicable date: Not applicable.

---

DOCUMENTS INCORPORATED
BY REFERENCE

The
registrants Reports on Form 8-K concerning the registrants WaMu Mortgage
Pass-Through Certificates, Series 2002-S1 (Certificates) issued pursuant to
the Pooling and Servicing Agreement dated as of January 1, 2002 (the Pooling
Agreement) are incorporated by reference as set forth in Part IV, Item 15.
Capitalized terms used herein but not otherwise defined shall have the meanings assigned
to them in the Pooling Agreement.

Part I

Item 1. Business.

Not
applicable.

Item 2. Properties.

Not applicable.

Item 3. Legal Proceedings.

There
are no material pending legal proceedings involving the mortgage loans related to the
Certificates, the trustee under the Pooling Agreement or the registrant with respect to
the mortgage loans, other than ordinary routine litigation incidental to the
trustees or the registrants duties under the Pooling Agreement.

Item 4. Submission of Matters to a
Vote of Security Holders.

No
vote or consent of holders of Certificates has been solicited for any purpose during 2002.

Part II

Item 5. Market for Registrants
Common Equity and Related Stockholder Matters.

Not
applicable.

Item 6. Selected Financial Data.

Not applicable.

Item 7. Managements Discussion
and Analysis of Financial Condition and Results of Operations.

Not
applicable.

Item 7A. Quantitative and Qualitative
Disclosures About Market Risk.

Not
applicable.

Item 8. Financial Statements and
Supplementary Data.

Not
applicable.

Item 9. Changes in and Disagreements
With Accountants on Accounting and Financial Disclosure.

No
material disagreement concerning accounting procedures or change of accountants has
occurred.

Part III

Item 10. Directors and Executive
Officers of the Registrant.

Not
applicable.

Item 11. Executive Compensation.

Not applicable.

Item 12. Security Ownership of
Certain Beneficial Owners and Management.

Not
applicable.

Item 13. Certain Relationships and
Related Transactions.

Not
applicable.

Item 14. Controls and Procedures.

Not
applicable.

Part IV

Item 15. Exhibits, Financial
Statement Schedules, and Reports on Form 8-K.

The
Pooling Agreement requires an officers certificate to be delivered to the Trustee on
or before April 30, 2003 stating as to the signer thereof, that (i) a review of the
activities of the Master Servicer during the calendar year ended December 31, 2002 and
performance under the Pooling Agreement has been made under such officers
supervision, and (ii) to the best of such officers knowledge, based on such review,
the Master Servicer has fulfilled all its obligations under the Pooling Agreement
throughout such year, or if there has been a default in the fulfillment of any such
obligation, specifying each such default known to such officer and the nature and status
thereof. Such officers certificate dated as of March 21st, 2003 is
attached hereto as Exhibit 99.1.

Attached hereto
as Exhibit 99.2 is the Certification of Disclosure for this Form 10-K.

The
Pooling Agreement requires a statement from a firm of independent public accountants to be
furnished to the Trustee on or before April 30, 2003, conducted in accordance with
attestation standards established by the American Institute of Certified Public
Accountants on the Master Servicers minimum servicing standards as a master servicer
as of and for the year ended December 31, 2002.  Such independent public accountant ́s statement, dated February 20, 2003
is attached hereto as Exhibit 99.3.  The management ́s assetrtion concerning compliance with Washington Mutual
Mortgage Securities Corp.s minimum servicing standards referenced therein is attached hereto as Exhibit 99.4.

The
registrant filed Reports on Form 8-K that described distributions made to
Certificateholders on the Distribution Date occurring in the month preceding the month in
which such Report on Form 8-K was filed and also provided certain information regarding
delinquent Mortgage Loans and credit enhancements as of such Distribution Date. Such
Reports on Form 8-K are incorporated by reference as follows:

WaMu Mortgage Pass-Through
Certificates, Series 2002-S1

|                                                       |                   
|------------------------------------------------------ | ------------------
|Monthly Distribution Report for month indicated - 2002 | Date Filed        
|
|
|March                                                  | March 11, 2002    
|April                                                  | April 10, 2002    
|May                                                    | May 9, 2002       
|June                                                   | June 7, 2002      
|July                                                   | July 9, 2002      
|August                                                 | August 8, 2002    
|September                                              | September 10, 2002
|October                                                | October 7, 2002   
|November                                               | November 8, 2002  
|December                                               | December 9, 2002  

---

SIGNATURES

Pursuant
to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the undersigned,
thereunto duly authorized.

Dated:      
March 26, 2003

| |                                                                                                                                                                              
| | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| | WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
By: /s/ Thomas G. Lehmann
------------------------------------------------
Thomas G. Lehmann
First Vice President and Sr. Counsel

Pursuant
to the requirements of the Securities Exchange Act of 1934, this report has been signed
below by the following persons on behalf of the registrant and in the capacities and on
the dates indicated.

By: /s/ Megan M. Davidson
----------------------------------------------------------------------------------
Megan M. Davidson, Senior Vice President and Director Date: March 20, 2003

By: /s/ Craig S. Davis
----------------------------------------------------------------------------------
Craig S. Davis, Director Date: March 21, 2003

By: /s/ Marangal I. Domingo
----------------------------------------------------------------------------------
Marangal I. Domingo, Chief Executive Officer and Director Date: March 21, 2003

By: /s/ Michael J. Kula
----------------------------------------------------------------------------------
Michael J. Kula, Senior Vice President and Director Date: March 21, 2003

By: /s/ Lehr Stuart
----------------------------------------------------------------------------------
Lehr Stuart, Director Date: March 24, 2003

By: /s/ Robert H. Miles
----------------------------------------------------------------------------------
Robert H. Miles, Senior Vice President and Director Date: March 21, 2003

By: /s/ Michael L. Parker
----------------------------------------------------------------------------------
Michael L. Parker, President and Director Date: March 21, 2003

By: /s/ Marc K. Malone
----------------------------------------------------------------------------------
Marc K. Malone, First Vice President and Controller Date: March 26, 2003

---

|Exhibit 99.1 |  | Officers Certificate as to Compliance for the WaMu Mortgage Pass-Through
Certificates, Series 2002-S1 dated as of March 21st, 2003
|------------ |  | -----------------------------------------------------------------------------------------------------------------------------------

WASHINGTON MUTUAL
MORTGAGE SECURITIES CORP. OFFICERS CERTIFICATE

The
undersigned officer of Washington Mutual Mortgage Securities Corp., a Delaware corporation
(the Company) hereby certifies on behalf of the Company and on his own behalf
for purposes of the WaMu Mortgage Pass-Through Certificates, Series 2002-S1, as follows:

| | 1. | I am the duly appointed, qualified and acting President of the Company.
| | -- | -----------------------------------------------------------------------

| | 2. | Capitalized terms used and not defined herein shall have the meanings ascribed
                    to such terms in the Pooling and Servicing Agreement related to the
                    above-referenced series of Certificates.
| | -- | -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| | 3. | I am duly authorized to execute and deliver this Officers Certificate on
                    behalf of the Company.
| | -- | --------------------------------------------------------------------------------------------------------------------

| | 4. | A review of the activities of the Master Servicer during the preceding calendar
                    year and performance under this Agreement has been made under my supervision.
| | -- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| | 5. | To the best of my knowledge, based on such review, the Master Servicer has
                    fulfilled all its obligations under the Agreement throughout such year.
| | -- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------

IN
WITNESS WHEREOF, I have signed my name as of March 21st, 2003.

| |                                                                                                                                                                
| | ---------------------------------------------------------------------------------------------------------------------------------------------------------------
| | WASHINGTON MUTUAL MORTGAGE SECURITIES CORP.
By: /s/ Michael L. Parker
------------------------------------------------
Michael L. Parker
President and Director

---

Exhibit 99.2

WAMU MORTGAGE
PASS-THROUGH CERTIFICATES, SERIES 2002-S1

I,
Michael L. Parker, a President and Director of Washington Mutual Mortgage Securities
Corp., certify that:

1.       
          I have reviewed this annual report on Form 10-K, and all reports on Form 8-K
          containing distribution or servicing reports filed in respect of periods
          included in the year covered by this annual report, of WaMu Mortgage
          Pass-Through Certificates, Series 2002-S1;

2.       
          Based on my knowledge, the information in these reports, taken as a whole, does
          not contain any untrue statement of a material fact or omit to state a material
          fact necessary to make the statements made, in light of the circumstances under
          which such statements were made, not misleading as of the last day of the period
          covered by this annual report;

3.       
          Based on my knowledge, the distribution or servicing information required to be
          provided to the trustee by the servicer under the pooling and servicing, or
          similar, agreement, for inclusion in these reports is included in these reports;

4.       
          I am responsible for reviewing the activities performed by the servicer under
          the pooling and servicing, or similar, agreement and based upon my knowledge and
          the annual compliance review required under that agreement, and except as
          disclosed in the reports, the servicer has fulfilled its obligations under that
          agreement; and

5.       
          The reports disclose all significant deficiencies relating to the
          servicers compliance with the minimum servicing standards based upon the
          report provided by an independent public accountant, after conducting a review
          in compliance with the Uniform Single Attestation Program for Mortgage Bankers
          or similar procedure, as set forth in the pooling and servicing, or similar,
          agreement, that is included in these reports.

In
giving the certifications above, I have reasonably relied on information provided to me by
unaffiliated third parties acting as servicers and sub-servicers.

Dated: March
21st, 2003

| |                                                                                                                    
| | -------------------------------------------------------------------------------------------------------------------
| | By: /s/ Michael L. Parker
------------------------------------------------
Michael L. Parker
President and Director

---

Exhibit 99.3

[LETTERHEAD OF DELOITTE & TOUCHE]

# INDEPENDENT
ACCOUNTANTS REPORT

Board of Directors and
StockholderWashington Mutual Mortgage Securities Corp.

We have examined managements
assertion, included in the accompanying Managements Assertion Concerning Compliance
with Washington Mutual Mortgage Securities Corp.s Minimum Servicing Standards, that
Washington Mutual Mortgage Securities Corp. (WMMSC) complied with the minimum servicing
standards in its role as Master Servicer as of and for the year ended December 31, 2002.
Such assertions were examined relating to those series of certificates included in the
attached Exhibit 1. Management is responsible for WMMSCs compliance with those
minimum servicing standards. Our responsibility is to express an opinion on
managements assertion about WMMSCs compliance based on our examination.

Our examination was conducted in
accordance with attestation standards established by the American Institute of Certified
Public Accountants and, accordingly, included examining, on a test basis, evidence about
WMMSCs compliance with its minimum servicing standards and performing such other
procedures as we considered necessary in the circumstances. Series of certificates subject
to such procedures were selected using sampling methods, and accordingly, we make no
representations that our examination procedures were performed on a specific series of
certificates as listed in the attached Exhibit I. We believe that our examination provides
a reasonable basis for our opinion. Our examination does not provide a legal determination
on WMMSCs compliance with its minimum servicing standards.

In our opinion, managements
assertion that WMMSC complied with the aforementioned minimum servicing standards as of
and for the year ended December 31, 2002, is fairly stated, in all material respects.

/s/  Deloitte & Touche LLP

February 20, 2003

---

# WASHINGTON MUTUAL
MORTGAGE SECURITIES CORP.

Exhibit I

---

|POOL NUMBER | SERIES NAME
|----------- | -----------
|1990        |            
|1663        |            
|1700        |            
|1707        |            
|1783        |            
|1795        |            
|1807        |            
|1709        |            
|1723        |            
|1710        |            
|1523        |            
|1681        |            
|1535        |            
|1655        |            
|1445        |            
|1507        |            
|1508        |            
|1546        |            
|1522        |            
|1776        |            
|1773        |            
|1797        |            
|1855        |            
|1732        |            
|1738        |            
|1867        |            
|1875        |            
|1898        |            
|1935        |            
|1949        |            
|1976        |            
|1992        |            
|2009        |            
|1828        |            
|1831        |            
|1836        |            
|1823        |            
|1870        |            
|1896        |            
|1939        |            
|1952        |            
|1971        |            
|2001        |            
|1842        |            
|2030        |            
|2033        |            
|1765        |            
|1851        |            
|1861        |            
|1857        |            
|1677        |            
|1740        |            
|1764        |            
|1705        |            
|1946        |            
|1001        |            
|1003        |            
|1007        |            
|1124        |            
|1122        |            
|1119        |            
|1120        |            
|1123        |            
|1104        |            
|1111        |            
|1112        |            
|1133        |            
|1126        |            
|1136        |            
|1138        |            
|1139        |            
|1140        |            
|1146        |            
|1145        |            
|1147        |            
|1149        |            
|1150        |            
|1127        |            
|1152        |            
|1157        |            
|1158        |            
|1128        |            
|1130        |            
|1125        |            
|1132        |            
|1134        |            
|1135        |            
|1156        |            
|1163        |            
|1159        |            
|1167        |            
|1168        |            
|1165        |            
|1166        |            
|1170        |            
|1160        |            
|1217        |            
|1218        |            
|1228        |            
|1252        |            
|1216        |            
|1258        |            
|1256        |            
|1259        |            
|1263        |            
|1221        |            
|1224        |            
|1225        |            
|1229        |            
|1248        |            
|1254        |            
|1233        |            
|1242        |            
|1243        |            
|1245        |            
|1241        |            
|1283        |            
|1253        |            
|1276        |            
|1279        |            
|1280        |            
|1281        |            
|1286        |            
|1287        |            
|1288        |            
|1290        |            
|1251        |            
|1262        |            
|1269        |            
|1270        |            
|1273        |            
|1275        |            
|1272        |            
|1294        |            
|1257        |            
|1295        |            
|1293        |            
|1308        |            
|1297        |            
|1312        |            
|1324        |            
|1349        |            
|1360        |            
|1351        |            
|1354        |            
|1355        |            
|1356        |            
|1359        |            
|1370        |            
|1376        |            
|1116        |            
|1323        |            
|1470        |            
|1545        |            
|1775        |            
|1788        |            
|1782        |            
|1800        |            
|1817        |            
|1813        |            
|1824        |            
|1844        |            
|1853        |            
|1863        |            
|1864        |            
|1811        |            
|1799        |            
|1931        |            
|1932        |            
|1938        |            
|1943        |            
|1944        |            
|1958        |            
|1959        |            
|1956        |            
|1981        |            
|2011        |            
|1848        |            
|1888        |            
|1889        |            
|1960        |            
|2006        |            
|1317        |            
|1318        |            
|1319        |            
|1321        |            
|1334        |            
|1340        |            
|1343        |            
|1345        |            
|1330        |            
|1342        |            
|1353        |            
|1380        |            
|1412        |            
|1458        |            
|1471        |            
|1475        |            
|1488        |            
|1494        |            
|1462        |            
|1466        |            
|1538        |            
|1502        |            
|1569        |            
|1632        |            
|1641        |            
|1510        |            
|1517        |            
|1528        |            
|1547        |            
|1549        |            
|1563        |            
|1562        |            
|1659        |            
|1650        |            
|1664        |            
|1671        |            
|1678        |            
|1682        |            
|1680        |            
|1689        |            
|1694        |            
|1696        |            
|1699        |            
|1727        |            
|1734        |            
|1729        |            
|1746        |            
|1743        |            
|1750        |            
|1753        |            
|1759        |            
|1772        |            
|1779        |            
|1790        |            
|1802        |            
|1777        |            
|1796        |            
|1749        |            
|1758        |            
|1818        |            
|1833        |            
|1846        |            
|1858        |            
|1814        |            
|1825        |            
|1843        |            
|1726        |            
|1748        |            
|1984        |            
|6215        |            
|6216        |            
|1945        |            
|1839        |            
|1882        |            
|1979        |            
|2003        |            
|1987        |            
|1991        |            
|1878        |            
|1893        |            
|1947        |            
|1967        |            
|1965        |            
|2023        |            
|2021        |            
|2016        |            
|2020        |            
|1762        |            
|1886        |            
|1942        |            
|1995        |            
|1996        |            
|1933        |            
|1363        |            
|1361        |            
|1362        |            
|1364        |            
|1366        |            
|1367        |            
|1368        |            
|1365        |            
|1369        |            
|1372        |            
|1374        |            
|1373        |            
|1371        |            
|1381        |            
|1385        |            
|1426        |            
|1377        |            
|1375        |            
|1384        |            
|6028        |            
|6023        |            
|1378        |            
|1379        |            
|1383        |            
|1391        |            
|1387        |            
|1388        |            
|1428        |            
|1430        |            
|1394        |            
|1432        |            
|1402        |            
|1405        |            
|1397        |            
|1406        |            
|1416        |            
|1437        |            
|1440        |            
|1448        |            
|1452        |            
|1434        |            
|1683        |            
|1757        |            
|1982        |            
|1978        |            
|1154        |            
|6517        |            
|6515        |            
|6516        |            
|1704        |            
|6217        |            
|1701        |            
|1702        |            
|1492        |            
|1711        |            

---

Exhibit 99.4

# Managements
Assertion Concerning Compliance with Washington Mutual Mortgage
Securities Corp.s Minimum Servicing Standards

February 20, 2003

Deloitte & Touche LLP700
Fifth Avenue Suite 4500 Seattle, Washington 98104

For the year ended December 31, 2002,
Washington Mutual Mortgage Securities Corp. (WMMSC) has complied in all material respects
with our minimum servicing standards set forth below under master servicing arrangements.
Primary servicing functions are performed by various subservicers.

Our minimum standards are:

I.    
          RECONCILIATIONS

A.    
          Reconciliations shall be prepared on a monthly basis for all clearing accounts.
          These reconciliations shall:

1)       
          Be mathematically accurate.

2)       
 Be prepared within forty-five (45) calendar days after each clearing
                    accounts statement cutoff date.

3)       
 Be reviewed and approved by someone other than the person who prepared the reconciliation, and document explanations for reconciling items.

B.    
          Reconciling items shall be resolved within ninety (90) calendar days of their
          original identification.

C.    
          Each clearing account shall be maintained as prescribed by applicable pooling
          and servicing agreements.

Exception  During the
year, certain reconciliations were not completed within our minimum servicing standards
and contained items that were not resolved within 90 calendar days of their original
identification. These occurrences did not adversely impact trustees, paying agents, or
investors. The reconciliation process has since been enhanced and the December 31, 2002
reconciliations were in compliance with the minimum servicing standards in all material
respects.

II.    
          SUBSERVICER REMITTANCES

A.    
          Remittances for mortgage payments and payoffs received from subservicers shall
          be deposited into the applicable clearing account within one business day of
          receipt.

B.    
          Remittances from subservicers shall be reconciled to applicable loan records
          maintained by WMMSC during the appropriate accounting cycle.

C.    
          Reconciliations shall be performed monthly for each subservicer remittance.
          These reconciliations shall:

1)       
          Be mathematically accurate.

2)       
          Be prepared within thirty (30) calendar days after the cutoff date.

III.    
          DISBURSEMENTS

A.    
          Disbursements to trustees, paying agents, and investors shall be made in
          accordance with the applicable pooling and servicing agreements and/or the
          prospectus indicating how cash flows are to be allocated.

B.    
          Disbursements from the clearing accounts for amounts remitted to trustees,
          paying agents and investors, per our monthly reports to those entities, shall
          agree with the bank statements for the clearing accounts.

C.    
          Only permitted withdrawals, per our interpretation of the applicable pooling and
          servicing agreements, shall be made from the clearing accounts.

D.    
          Disbursements to trustees, paying agents and investors from clearing accounts,
          made via wire transfer, shall be made only by authorized personnel.

IV.    
          INVESTOR ACCOUNTING AND REPORTING

A.    
          Statements to the trustees, paying agents and investors shall be provided with
          each monthly distribution, in accordance with applicable pooling and servicing
          agreements, detailing the applicable distribution activity and effect on the
          unpaid principal balance of the mortgage loans.

V.    
          MORTGAGOR LOAN ACCOUNTING

A.    
          Loan records maintained by WMMSC shall agree with, or reconcile within material
          amounts to, the mortgage loan records maintained by the subservicer with respect
          to unpaid principal balance on a monthly basis.

VI.    
          DELIQUENCIES

A.    
          Reports from subservicers identifying delinquent loans shall be received and
          reviewed monthly. The number and aggregate principal balances of delinquent
          mortgage loans contained in these reports shall be included in our monthly
          statements to the trustees, paying agents and investors

VII.    
          INSURANCE POLICIES

As of and for this same period,
Washington Mutual, Inc. (the ultimate parent of WMMSC) maintained a fidelity bond in the
amount $110,000,000 and a mortgage impairment/mortgagees errors and omissions and
professional liability insurance policy in the amount of $20,000,000.

/s/ Michael L. Parker 
____________________________________Michael L. ParkerPresident Washington Mutual
Mortgage Securities Corp.

/s/ Marangal I. Domingo 
____________________________________Marangal I. DomingoDirector
and Chief Executive OfficerWashington Mutual Mortgage Securities Corp.