The use of alcohol and recreational drugs in relationship to sexual practices was investigated longitudinally and cross-sectionally in a cohort of homosexually active men at a Boston community health center. Use of marijuana, nitrite inhalants, and cocaine decreased by approximately 25-48% during 42 months of follow-up, whereas use of alcohol showed little change. Men who initially reported both high-risk sexual practices and some use of alcohol or marijuana and who subsequently stopped using marijuana or reduced their frequency of alcohol use were significantly more likely to stop those unsafe sexual practices than were those who continued to use these substances. Men who at their most recent visit reported impaired judgement during sexual activity due to substance use, and particularly due to drug use, had significantly higher levels of perceived susceptibility to AIDS and barriers to behavior change, and lower levels of self-efficacy, which were independent of their unsafe sexual practices.