-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 IUTmnTvQ0qsMA/2Epi/2cOuwocENcGSxWG6r7uUlOk0b7jdeJnkIxDyZplmEiE6P
 zhRcKq/XetmGOJM+pOs6uw==

<SEC-DOCUMENT>0001057352-03-000009.txt : 20030904
<SEC-HEADER>0001057352-03-000009.hdr.sgml : 20030904
<ACCEPTANCE-DATETIME>20030904173802
ACCESSION NUMBER:		0001057352-03-000009
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20030904
FILED AS OF DATE:		20030904

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			COSTAR GROUP INC
		CENTRAL INDEX KEY:			0001057352
		STANDARD INDUSTRIAL CLASSIFICATION:	SERVICES-BUSINESS SERVICES, NEC [7389]
		IRS NUMBER:				522091508
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		2 BETHESDA METRO CENTER
		STREET 2:		10TH FLOOR
		CITY:			BETHESDA
		STATE:			MD
		ZIP:			20814
		BUSINESS PHONE:		3012158300

	MAIL ADDRESS:	
		STREET 1:		2 BETHESDA METRO CENTER
		CITY:			BETHESDA
		STATE:			MD
		ZIP:			20814

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			NASSETTA CHRISTOPHER J
		CENTRAL INDEX KEY:			0001078095

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-24531
		FILM NUMBER:		03882201

	BUSINESS ADDRESS:	
		STREET 1:		77 WEST WACKER DR
		STREET 2:		STE 3900
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60601
		BUSINESS PHONE:		3129171300

	MAIL ADDRESS:	
		STREET 1:		77 WEST WACKER DRIVE
		STREET 2:		SUITE 3900
		CITY:			CHICAGO
		STATE:			IL
		ZIP:			60601
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-09-04</periodOfReport>
    <issuer>
        <issuerCik>0001057352</issuerCik>
        <issuerName>COSTAR GROUP INC</issuerName>
        <issuerTradingSymbol>CSGP</issuerTradingSymbol></issuer><reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001078095</rptOwnerCik>
            <rptOwnerName>NASSETTA CHRISTOPHER J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>HOST MARRIOTT CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>6903 ROCKLEDGE DRIVE, SUITE 1500</rptOwnerStreet2>
            <rptOwnerCity>BETHESDA</rptOwnerCity>
            <rptOwnerState>MD</rptOwnerState>
            <rptOwnerZipCode>20817</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><derivativeTable><derivativeTransaction><securityTitle><value>Options to acquire Common Stock</value></securityTitle><conversionOrExercisePrice><value>30.061</value></conversionOrExercisePrice><transactionDate><value>2003-09-04</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>A</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>5000</value><footnoteId id="F1"/></transactionShares><transactionPricePerShare><footnoteId id="F2"/></transactionPricePerShare><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>2004-09-04</value><footnoteId id="F1"/></exerciseDate><expirationDate><value>2013-09-03</value></expirationDate><underlyingSecurity><underlyingSecurityTitl
e><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>5000</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>10000</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value/></natureOfOwnership></ownershipNature></derivativeTransaction></derivativeTable><footnotes><footnote id="F1">Vests over 4 years (1250 on 9/4/2004, 1250 on 9/4/2005, 1250 on 9/4/2006, 1250 on 9/4/2007)</footnote><footnote id="F2">Not applicable</footnote></footnotes><remarks/><ownerSignature><signatureName>Christopher Nassetta</signatureName><signatureDate>2003-09-04</signatureDate></ownerSignature></ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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