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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-12-12</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000939563</rptOwnerCik>
            <rptOwnerName>GEARY RONALD G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2700 GAST BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>LOUISVILLE</rptOwnerCity>
            <rptOwnerState>KY</rptOwnerState>
            <rptOwnerZipCode>40205</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>2012-12-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>52.48</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>23164.697</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-12-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>55.13</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>25664.697</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-12-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>7500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>64.2676</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18164.697</value>
                </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>52.48</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-12-12</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>2011-01-01</value>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2021-01-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>55.13</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-12-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2012-01-01</value>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <value>2022-01-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>2500</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2500</value>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">On December 12, 2012, the Reporting Person transmitted to the Securities and Exchange Commission (the &quot;SEC&quot;) a Form 144 covering the sale of the Issuer's common stock reported in Table I. The price reported in column 4 is a weighted average price. These shares were sold by the Reporting Person in multiple transactions at prices ranging from $64.2649 to $64.31, inclusive. The Reporting Person undertakes to provide Issuer, any security holder of Issuer or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (1).</footnote>
        <footnote id="F2">These options were part of a previously reported grant of 5,000 on January 1, 2011, by the Issuer to the Reporting Person that vested in two equal installments beginning on January 1, 2011.</footnote>
        <footnote id="F3">These options were part of a previously reported grant of 5,000 on January 1, 2012, by the Issuer to the Reporting Person that vested or will vest in two equal installments beginning on January 1, 2012.</footnote>
        <footnote id="F4">Represents total number of unexercised stock options held by the Reporting Person as of December 12, 2012.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Ronald G. Geary, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2012-12-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2012-12-12
0000740260
VENTAS INC
VTR
0000939563
GEARY RONALD G
2700 GAST BLVD.
LOUISVILLE
KY
40205
1
0
0
0
Common Stock
2012-12-12
4
M
0
5000
52.48
A
23164.697
D
Common Stock
2012-12-12
4
M
0
2500
55.13
A
25664.697
D
Common Stock
2012-12-12
4
S
0
7500
64.2676
D
18164.697
D
Stock Option (Right to Buy)
52.48
2012-12-12
4
M
0
5000
0
D
2011-01-01
2021-01-01
Common Stock
5000
5000
D
Stock Option (Right to Buy)
55.13
2012-12-12
4
M
0
2500
0
D
2012-01-01
2022-01-01
Common Stock
2500
2500
D
On December 12, 2012, the Reporting Person transmitted to the Securities and Exchange Commission (the "SEC") a Form 144 covering the sale of the Issuer's common stock reported in Table I. The price reported in column 4 is a weighted average price. These shares were sold by the Reporting Person in multiple transactions at prices ranging from $64.2649 to $64.31, inclusive. The Reporting Person undertakes to provide Issuer, any security holder of Issuer or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (1).
These options were part of a previously reported grant of 5,000 on January 1, 2011, by the Issuer to the Reporting Person that vested in two equal installments beginning on January 1, 2011.
These options were part of a previously reported grant of 5,000 on January 1, 2012, by the Issuer to the Reporting Person that vested or will vest in two equal installments beginning on January 1, 2012.
Represents total number of unexercised stock options held by the Reporting Person as of December 12, 2012.
Ronald G. Geary, By: T. Richard Riney, Attorney-In-Fact
2012-12-14