Rates of drug use among gay men are higher than their heterosexual counterparts and drug use is a prominent risk factor for HIV transmission. Studies using heterosexual samples have found that being partnered reduces the risk of drug use and individuals in a relationship tend to have similar use patterns. Studies among gay men suggest that sexual agreements may be an important predictor of drug use. Data from 322 partnered gay men were collected and the 161 matched couples were categorized as monogamous (52.8 %), monogamish (14.9 %), open (13.0 %), and discrepant (19.3 %). Patterns of significance and significant trends suggested that monogamous men reported lower rates of marijuana and other drug use compared to open and monogamish men. Men in discrepant relationships did not differ from any other group. Partners' marijuana and other drug use was significantly interdependent in the overall sample; however, substantial variation in the magnitude and significance of interdependence was observed across sexual arrangement categories. Sexual arrangement and the use of drugs during sex both contributed to the prediction of UAI with casual partners among non-monogamous men. Implications for substance use treatment and HIV prevention are discussed.