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

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-01-01</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001203367</rptOwnerCik>
            <rptOwnerName>LUSTIG MATTHEW J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>30 ROCKEFELLER PLAZA</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK,</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10020</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>2012-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>340.105</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>55.13</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3812.263</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1028</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4848.485</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>24770619</value>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote (4)(5)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>55.13</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2012-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                    <footnoteId id="F6"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2012-01-01</value>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <value>2022-01-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>5000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>8191</value>
                    <footnoteId id="F8"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Common stock in the form of Units granted under the Ventas Nonemployee Director 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. 2006 Stock Plan for Directors (the &quot;Stock 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 Stock Plan. These Restricted Stock Units vest in two equal annual installments beginning on the first anniversary of the date of grant.</footnote>
        <footnote id="F3">Includes 8.222 Units acquired on December 30, 2011 under the Plan as a result of dividend equivalents credited with respect to the dividend on Issuer's common stock paid on December 30, 2011.</footnote>
        <footnote id="F4">Prometheus Senior Quarters LLC, a Delaware limited liability company (&quot;Prometheus&quot;), Lazard Senior Housing Partners LP, a Delaware limited partnership (&quot;LSHP&quot;), and LSHP Coinvestment Partnership I LP, a Delaware limited partnership (&quot;LSHP Coinvest&quot;), collectively, may be deemed to beneficially own 24,770,619 shares of common stock. The Managing Members of Prometheus are LFSRI II-CADIM Alternative Partnership L.P., LF Strategic Realty Investors II L.P. and LFSRI II Alternative Partnership L.P., each a Delaware limited partnership, and Lazard Freres Real Estate Investors L.L.C., a New York limited liability company, (&quot;LFREI&quot;) is the general partner of each such limited partnership. Lazard Senior Housing Partners GP LLC, a Delaware limited liability company (&quot;LSHP GP&quot;), is the general partner of LSHP. LSHP Coinvestment I GP LLC, a Delaware limited liability company (&quot;LSHP Coinvest GP&quot;), is the general partner of LSHP Coinvest.</footnote>
        <footnote id="F5">[Continued from previous] Lazard Alternative Investments LLC, a Delaware limited liability company (&quot;LAI&quot;), is the managing member of each of LFREI, LSHP GP and LSHP Coinvest GP. In his capacity as Managing Director of LAI, Managing Principal of LFREI and Managing Principal and Chief Executive Officer of each of LSHP GP and LSHP Coinvest GP, Mr. Lustig may be deemed to be the beneficial owner of the securities reported herein. Mr. Lustig expressly disclaims beneficial ownership of the securities reported herein, except to the extent of his pecuniary interest therein, if any.</footnote>
        <footnote id="F6">Stock options granted by Issuer pursuant to the Stock Plan.</footnote>
        <footnote id="F7">These options vest in two equal annual installments beginning on the date of grant.</footnote>
        <footnote id="F8">Represents total number of unexercised stock options held by the Reporting Person as of January 1, 2012.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Matthew J. Lustig, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2012-01-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "edgar.xml", "description": "PRIMARY DOCUMENT", "secsgml_size_bytes": 10714}]
X0304
4
2012-01-01
0000740260
VENTAS INC
VTR
0001203367
LUSTIG MATTHEW J
30 ROCKEFELLER PLAZA
NEW YORK,
NY
10020
1
0
0
0
Common Stock
2012-01-01
4
A
0
340.105
55.13
A
3812.263
D
Common Stock
2012-01-01
4
A
0
1028
0
A
4848.485
D
Common Stock
24770619
I
See Footnote (4)(5)
Stock Option (Right to Buy)
55.13
2012-01-01
4
A
0
5000
0
A
2012-01-01
2022-01-01
Common Stock
5000
8191
D
Common stock in the form of Units granted under the Ventas Nonemployee Director 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. 2006 Stock Plan for Directors (the "Stock Plan").  Such Units are payable solely in common stock and subject to the terms and conditions of the Director's deferral election and the Stock Plan. These Restricted Stock Units vest in two equal annual installments beginning on the first anniversary of the date of grant.
Includes 8.222 Units acquired on December 30, 2011 under the Plan as a result of dividend equivalents credited with respect to the dividend on Issuer's common stock paid on December 30, 2011.
Prometheus Senior Quarters LLC, a Delaware limited liability company ("Prometheus"), Lazard Senior Housing Partners LP, a Delaware limited partnership ("LSHP"), and LSHP Coinvestment Partnership I LP, a Delaware limited partnership ("LSHP Coinvest"), collectively, may be deemed to beneficially own 24,770,619 shares of common stock. The Managing Members of Prometheus are LFSRI II-CADIM Alternative Partnership L.P., LF Strategic Realty Investors II L.P. and LFSRI II Alternative Partnership L.P., each a Delaware limited partnership, and Lazard Freres Real Estate Investors L.L.C., a New York limited liability company, ("LFREI") is the general partner of each such limited partnership. Lazard Senior Housing Partners GP LLC, a Delaware limited liability company ("LSHP GP"), is the general partner of LSHP. LSHP Coinvestment I GP LLC, a Delaware limited liability company ("LSHP Coinvest GP"), is the general partner of LSHP Coinvest.
[Continued from previous] Lazard Alternative Investments LLC, a Delaware limited liability company ("LAI"), is the managing member of each of LFREI, LSHP GP and LSHP Coinvest GP. In his capacity as Managing Director of LAI, Managing Principal of LFREI and Managing Principal and Chief Executive Officer of each of LSHP GP and LSHP Coinvest GP, Mr. Lustig may be deemed to be the beneficial owner of the securities reported herein. Mr. Lustig expressly disclaims beneficial ownership of the securities reported herein, except to the extent of his pecuniary interest therein, if any.
Stock options granted by Issuer pursuant to the Stock Plan.
These options vest in two equal annual installments beginning on the date of grant.
Represents total number of unexercised stock options held by the Reporting Person as of January 1, 2012.
Matthew J. Lustig, By: T. Richard Riney, Attorney-In-Fact
2012-01-04