Adolescent and young adult males who have sex with men (MSM) remain at high risk of HIV infection. Many sexual risk factors have been identified, yet the role of substance use remains controversial. We assess the extent to which urban young MSM report being "high" on drugs or alcohol during sex and the association between being "high" and unprotected anal intercourse (UAI). During summer 2000, 3,075 MSM aged 15-25 years completed a 20-minute interview for the Community Intervention Trial for Youth Project. Participants were asked about their last sexual contact with main and nonmain partners, including whether they were "high on drugs or alcohol." 18.6% of MSM with a main partner reported being high during their last sexual encounter; 25.0% reported UAI. Among men with a nonmain partner, 29.3% reported being high, and 12.3% reported UAI. Being high was associated with unprotected receptive anal intercourse with nonmain partners (odds ratio = 1.66, p = .02). HIV prevention should include messages about the potential dangers of drinking and drug use in situations where sexual encounters with nonmain partners may occur.