A sample of San Francisco gay/bisexual men in substance-abuse treatment (N = 314) was compared to the San Francisco Men's Health Study (SFMHS) cohort to compare levels of sexual behaviors that are high-risk for HIV transmission. Quantitative data were supplemented by analysis of transcripts of focus group discussions with gay men in treatment at the same agency. Sexual risk for HIV infection was significantly higher for gay/bisexual men in substance-abuse treatment than among the community-based (SFMHS) sample of gay/bisexual men. Among those entering outpatient alcohol/drug treatment, 21% reported unprotected insertive and sex, 23% reported unprotected receptive anal sex, and 32% reported unprotected insertive and/or receptive anal sex during the previous 3 months. In comparison, in the SFMHS cohort, 17% reported unprotected insertive and sex, 15% reported unprotected receptive anal sex, and 22% reported unprotected insertive and/or receptive anal sex during the previous 6 months. Substance abusers in focus groups identified a number of factors that made it difficult to reduce their sexual risk behavior, including the perceived disinhibiting effects of alcohol and other drugs, learned patterns of association between substance use and sex (especially methamphetamine use and anal sex), low self-esteem, lack of assertiveness and negotiating skills, and perceived powerlessness.