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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-01-01</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001213928</rptOwnerCik>
            <rptOwnerName>GELLERT JAY M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>21650 OXNARD ST., 22ND FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WOODLAND HILLS,</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>91367</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>386.279</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>64.72</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>67841.578</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1545</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>69749.269</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Common stock in the form of units granted under the Ventas Nonemployee Directors' Deferred Stock Compensation Plan (the &quot;Plan&quot;) in lieu of director fees pursuant to the director's deferral election.  Such units are payable solely in common stock and subject to the terms and conditions of the director's deferral election and the Plan.</footnote>
        <footnote id="F2">Restricted stock units granted by Issuer pursuant to the Ventas, Inc. 2012 Incentive Plan (the &quot;Incentive Plan&quot;). Such units are payable solely in common stock and subject to the terms and conditions of the director's deferral election and the Incentive Plan. These restricted stock units vest in two equal annual installments beginning on the first anniversary of the date of the grant.</footnote>
        <footnote id="F3">Includes 162.805 units acquired on December 28, 2012 under the Plan as a result of dividend equivalents credited with respect to the dividend on Issuer's common stock paid on December 28, 2012.</footnote>
        <footnote id="F4">Includes 199.886 shares acquired on December 28, 2012 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F5">As of January 1, 2013, Reporting Person also owns options to purchase 45,000 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Jay M. Gellert, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2013-01-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-01-01
0000740260
VENTAS INC
VTR
0001213928
GELLERT JAY M
21650 OXNARD ST., 22ND FLOOR
WOODLAND HILLS,
CA
91367
1
0
0
0
Common Stock
2013-01-01
4
A
0
386.279
64.72
A
67841.578
D
Common Stock
2013-01-01
4
A
0
1545
0
A
69749.269
D
Common stock in the form of units granted under the Ventas Nonemployee Directors' Deferred Stock Compensation Plan (the "Plan") in lieu of director fees pursuant to the director's deferral election.  Such units are payable solely in common stock and subject to the terms and conditions of the director's deferral election and the Plan.
Restricted stock units granted by Issuer pursuant to the Ventas, Inc. 2012 Incentive Plan (the "Incentive Plan"). Such units are payable solely in common stock and subject to the terms and conditions of the director's deferral election and the Incentive Plan. These restricted stock units vest in two equal annual installments beginning on the first anniversary of the date of the grant.
Includes 162.805 units acquired on December 28, 2012 under the Plan as a result of dividend equivalents credited with respect to the dividend on Issuer's common stock paid on December 28, 2012.
Includes 199.886 shares acquired on December 28, 2012 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
As of January 1, 2013, Reporting Person also owns options to purchase 45,000 shares of Issuer's common stock.
Jay M. Gellert, By: T. Richard Riney, Attorney-In-Fact
2013-01-03