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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-07-01</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>353 N. CLARK STREET, SUITE 3300</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO,</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60654</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>2013-07-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>10434</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>68.61</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>110715.994</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </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 191.137 shares acquired between January 23, 2013 and July 2, 2013 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F3">Includes 603.215 shares acquired between March 28, 2013 and June 28, 2013 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F4">As of July 1, 2013, Reporting Person also owns options to purchase 59,883 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Todd W. Lillibridge, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2013-07-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-07-01
0000740260
VENTAS INC
VTR
0001494933
Lillibridge Todd W.
353 N. CLARK STREET, SUITE 3300
CHICAGO,
IL
60654
0
1
0
0
EVP, Medical Prop. Operations
Common Stock
2013-07-01
4
F
0
10434
68.61
D
110715.994
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 191.137 shares acquired between January 23, 2013 and July 2, 2013 under the Ventas Employee and Director Stock Purchase Plan.
Includes 603.215 shares acquired between March 28, 2013 and June 28, 2013 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
As of July 1, 2013, Reporting Person also owns options to purchase 59,883 shares of Issuer's common stock.
Todd W. Lillibridge, By: T. Richard Riney, Attorney-In-Fact
2013-07-03