-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
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<SEC-DOCUMENT>0000914190-09-000583.txt : 20090720
<SEC-HEADER>0000914190-09-000583.hdr.sgml : 20090719
<ACCEPTANCE-DATETIME>20090720183321
ACCESSION NUMBER:		0000914190-09-000583
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090716
FILED AS OF DATE:		20090720
DATE AS OF CHANGE:		20090720

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			BETTERLEY LAURENCE L
		CENTRAL INDEX KEY:			0001243511

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-52082
		FILM NUMBER:		09953904

	MAIL ADDRESS:	
		STREET 1:		C/O PLATO LEARNING, INC.
		STREET 2:		10801 NESBITT AVENUE SOUTH
		CITY:			BLOOMINGTON
		STATE:			MN
		ZIP:			55437

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Cardiovascular Systems Inc
		CENTRAL INDEX KEY:			0001180145
		STANDARD INDUSTRIAL CLASSIFICATION:	SURGICAL & MEDICAL INSTRUMENTS & APPARATUS [3841]
		IRS NUMBER:				411698056
		FISCAL YEAR END:			0630

	BUSINESS ADDRESS:	
		STREET 1:		651 CAMPUS DRIVE
		CITY:			ST. PAUL
		STATE:			MN
		ZIP:			55112
		BUSINESS PHONE:		651-259-1600

	MAIL ADDRESS:	
		STREET 1:		651 CAMPUS DRIVE
		CITY:			ST. PAUL
		STATE:			MN
		ZIP:			55112

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	REPLIDYNE INC
		DATE OF NAME CHANGE:	20020813
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-07-16</periodOfReport>

    <issuer>
        <issuerCik>0001180145</issuerCik>
        <issuerName>Cardiovascular Systems Inc</issuerName>
        <issuerTradingSymbol>CSII</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001243511</rptOwnerCik>
            <rptOwnerName>BETTERLEY LAURENCE L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>651 CAMPUS DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ST. PAUL</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55112</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Financial Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2009-07-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>23256</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>71781</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>8.75</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-03-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>14234</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14234</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Exercisable:  7,117 on 3/2/10 and 3/2/11.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ John R. Remakel as Attorney-in-Fact for Laurence L. Betterley pursuant to Power of Attorney previously filed.</signatureName>
        <signatureDate>2009-07-20</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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