<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-07-01</periodOfReport>

    <issuer>
        <issuerCik>0000740260</issuerCik>
        <issuerName>VENTAS INC</issuerName>
        <issuerTradingSymbol>VTR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001494933</rptOwnerCik>
            <rptOwnerName>Lillibridge Todd W.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>111 SOUTH WACKER DRIVE, SUITE 4800</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO,</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP, Medical Prop. Operations</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-07-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5242</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>53.74</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>109501.744</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares withheld to pay the taxes on the vesting of restricted stock granted on July 1, 2010 under the Ventas, Inc. 2006 Incentive Plan.</footnote>
        <footnote id="F2">Includes 134.744 shares acquired between  April 4, 2011 and July 5, 2011 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F3">As of July 1, 2011, Reporting Person also owns options to purchase 7,962 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Todd W. Lillibridge, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2011-07-06</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0303
4
2011-07-01
0000740260
VENTAS INC
VTR
0001494933
Lillibridge Todd W.
111 SOUTH WACKER DRIVE, SUITE 4800
CHICAGO,
IL
60606
0
1
0
0
EVP, Medical Prop. Operations
Common Stock
2011-07-01
4
F
0
5242
53.74
D
109501.744
D
Represents shares withheld to pay the taxes on the vesting of restricted stock granted on July 1, 2010 under the Ventas, Inc. 2006 Incentive Plan.
Includes 134.744 shares acquired between  April 4, 2011 and July 5, 2011 under the Ventas Employee and Director Stock Purchase Plan.
As of July 1, 2011, Reporting Person also owns options to purchase 7,962 shares of Issuer's common stock.
Todd W. Lillibridge, By: T. Richard Riney, Attorney-In-Fact
2011-07-06