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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-01-18</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>2013-01-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1287</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>65.75</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>111398.226</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-01-20</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>426</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>65.75</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>111717.642</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">Represents shares withheld to pay the taxes on the vesting of restricted stock granted on January 18, 2012 under the Ventas, Inc. 2006 Incentive Plan (the &quot;Plan&quot;).</footnote>
        <footnote id="F2">Represents shares withheld to pay the taxes on the vesting of restricted stock granted on January 20, 2011 under the Plan.</footnote>
        <footnote id="F3">Includes 237.656 shares acquired between July 3, 2012 and January 22, 2013 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F4">Includes 507.76 shares acquired between September 28, 2012 and December 28, 2012 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F5">As of January 20, 2013, Reporting Person also owns options to purchase 32,901 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Todd W. Lillibridge, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2013-01-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-01-18
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
2013-01-18
4
F
0
1287
65.75
D
111398.226
D
Common Stock
2013-01-20
4
F
0
426
65.75
D
111717.642
D
Represents shares withheld to pay the taxes on the vesting of restricted stock granted on January 18, 2012 under the Ventas, Inc. 2006 Incentive Plan (the "Plan").
Represents shares withheld to pay the taxes on the vesting of restricted stock granted on January 20, 2011 under the Plan.
Includes 237.656 shares acquired between July 3, 2012 and January 22, 2013 under the Ventas Employee and Director Stock Purchase Plan.
Includes 507.76 shares acquired between September 28, 2012 and December 28, 2012 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
As of January 20, 2013, Reporting Person also owns options to purchase 32,901 shares of Issuer's common stock.
Todd W. Lillibridge, By: T. Richard Riney, Attorney-In-Fact
2013-01-23