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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 BWilifLcShhH3lxYnkvN6W+j/2MeGxu7hIcKbSr2audwY1IxHJEJ0/LaaqiXptON
 JFnDCQFU5rQ1il4iKP/FoA==

<SEC-DOCUMENT>0001179110-10-004495.txt : 20100304
<SEC-HEADER>0001179110-10-004495.hdr.sgml : 20100304
<ACCEPTANCE-DATETIME>20100304164943
ACCESSION NUMBER:		0001179110-10-004495
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100302
FILED AS OF DATE:		20100304
DATE AS OF CHANGE:		20100304

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			COMFORT SYSTEMS USA INC
		CENTRAL INDEX KEY:			0001035983
		STANDARD INDUSTRIAL CLASSIFICATION:	ELECTRICAL WORK [1731]
		IRS NUMBER:				760526487
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		675 BERING DRIVE
		STREET 2:		SUITE 400
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77057
		BUSINESS PHONE:		7138309600

	MAIL ADDRESS:	
		STREET 1:		675 BERING DRIVE
		STREET 2:		SUITE 400
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77057

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Murdy William F
		CENTRAL INDEX KEY:			0001205063

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-13011
		FILM NUMBER:		10657781

	MAIL ADDRESS:	
		STREET 1:		COMFORT SYSTEMS USA, INC.
		STREET 2:		675 BERING DRIVE, SUITE 400
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77057

	FORMER NAME:	
		FORMER CONFORMED NAME:	MURDY WILLIAM F
		DATE OF NAME CHANGE:	20021109
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-03-02</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001035983</issuerCik>
        <issuerName>COMFORT SYSTEMS USA INC</issuerName>
        <issuerTradingSymbol>FIX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001205063</rptOwnerCik>
            <rptOwnerName>Murdy William F</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O COMFORT SYSTEMS USA, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>675 BERING DRIVE, SUITE 400</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77057</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Executive Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-03-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1000</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>12.25</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>293827</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Trust</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">These transactions were affected pursuant to a Rule 10b5-1 plan adopted by the Reporting Person in accordance with Rule 10b5-1 of the Securities Exchange Act of 1934, as amended.</footnote>
        <footnote id="F2">The average price of sold shares; full information regarding the number of shares sold and specific prices will be made available upon request to the Company's Office of the General Counsel.</footnote>
        <footnote id="F3">Shares held in trust by William Murdy Trust, trustee to William F. Murdy.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ William F. Murdy</signatureName>
        <signatureDate>2010-03-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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