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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-05-24</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001388433</rptOwnerCik>
            <rptOwnerName>Doman Timothy A</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>SVP-Asset Management</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-05-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4335</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>21.57</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>16206.402</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-05-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4335</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>54.9014</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11905.472</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3417</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>21.57</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2011-05-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4335</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2009-02-27</value>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-02-27</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4335</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11567</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On May 24, 2011, the Reporting Person transmitted to the Securities and Exchange Commission a Form 144 covering the sale of the Issuer's common stock reported on Table I.</footnote>
        <footnote id="F2">Includes 34.07 shares acquired between March 1, 2011 and May 25, 2011, under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F3">Reporting Person disclaims beneficial ownership of these 3,417 shares except to the extent of the Reporting Person's pecuniary interest in the shares.</footnote>
        <footnote id="F4">These options were part of a previously reported grant of 13,007 on February 27, 2009, by the Issuer to the Reporting Person that vested in three equal annual installments on February 27, 2009, February 27, 2010 and February 27, 2011.</footnote>
        <footnote id="F5">Represents total number of unexercised stock options held directly by the Reporting Person as of May 24, 2011. Reporting Person also owns a total of 13,355 unexercised stock options indirectly by trust.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Timothy A. Doman, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2011-05-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0303
4
2011-05-24
0000740260
VENTAS INC
VTR
0001388433
Doman Timothy A
111 SOUTH WACKER DRIVE, SUITE 4800
CHICAGO
IL
60606
0
1
0
0
SVP-Asset Management
Common Stock
2011-05-24
4
M
0
4335
21.57
A
16206.402
D
Common Stock
2011-05-24
4
S
0
4335
54.9014
D
11905.472
D
Common Stock
3417
I
By Trust
Stock Option (Right to Buy)
21.57
2011-05-24
4
M
0
4335
0
D
2009-02-27
2019-02-27
Common Stock
4335
11567
D
On May 24, 2011, the Reporting Person transmitted to the Securities and Exchange Commission a Form 144 covering the sale of the Issuer's common stock reported on Table I.
Includes 34.07 shares acquired between March 1, 2011 and May 25, 2011, under the Ventas Employee and Director Stock Purchase Plan.
Reporting Person disclaims beneficial ownership of these 3,417 shares except to the extent of the Reporting Person's pecuniary interest in the shares.
These options were part of a previously reported grant of 13,007 on February 27, 2009, by the Issuer to the Reporting Person that vested in three equal annual installments on February 27, 2009, February 27, 2010 and February 27, 2011.
Represents total number of unexercised stock options held directly by the Reporting Person as of May 24, 2011. Reporting Person also owns a total of 13,355 unexercised stock options indirectly by trust.
Timothy A. Doman, By: T. Richard Riney, Attorney-In-Fact
2011-05-26