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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-09-10</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001198222</rptOwnerCik>
            <rptOwnerName>ROSENBERG SHELI  Z</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>155 NORTH WACKER DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO,</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</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>2013-09-10</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>22.15</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>81058.724</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>22.15</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2013-09-10</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2004-01-05</value>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2014-01-05</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>40000</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Includes 1,432.74 shares acquired between January 3, 2013 and September 11, 2013 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F2">Includes 895.353 shares acquired between March 28, 2013 and June 28, 2013 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F3">These options were part of a previously reported grant of 5,000 on January 5, 2004 by the Issuer to the Reporting Person that vested in two equal annual installments beginning on January 5, 2004.</footnote>
        <footnote id="F4">Represents total number of unexercised stock options held by the Reporting Person as of September 10, 2013.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Sheli Z. Rosenberg, By T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2013-09-12</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-09-10
0000740260
VENTAS INC
VTR
0001198222
ROSENBERG SHELI  Z
155 NORTH WACKER DRIVE
CHICAGO,
IL
60606
1
0
0
0
Common Stock
2013-09-10
4
M
0
5000
22.15
A
81058.724
D
Stock Option (Right to Buy)
22.15
2013-09-10
4
M
0
5000
0
D
2004-01-05
2014-01-05
Common Stock
5000
40000
D
Includes 1,432.74 shares acquired between January 3, 2013 and September 11, 2013 under the Ventas Employee and Director Stock Purchase Plan.
Includes 895.353 shares acquired between March 28, 2013 and June 28, 2013 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
These options were part of a previously reported grant of 5,000 on January 5, 2004 by the Issuer to the Reporting Person that vested in two equal annual installments beginning on January 5, 2004.
Represents total number of unexercised stock options held by the Reporting Person as of September 10, 2013.
Sheli Z. Rosenberg, By T. Richard Riney, Attorney-In-Fact
2013-09-12