OBJECTIVE To determine if an association exists in young men who have sex with men (MSM) between being under the influence of alcohol or drugs during sex and participation in sexual behaviors which increase the risk of human immunodeficiency virus (HIV). STUDY DESIGN A total of 3492 young MSM were interviewed through the Young Men's Survey, an anonymous, cross-sectional, multisite, venue-based survey conducted from 1994 through 1998 at 194 public venues frequented by MSM aged 15 to 22 years in 7 US cities. RESULTS The majority of young MSM reported both receptive and insertive anal intercourse, and of these, approximately half reported not using condoms. Report of unprotected receptive anal intercourse at least once in the prior 6 months was associated with being under the influence of alcohol (adjusted odds ratio [AOR]=1.5; 95% confidence interval [CI]=1.2-1.8), cocaine (AOR=1.6; 95% CI=1.1-2.2), amphetamines (AOR=1.5; 95% CI=1.1-2.0) or marijuana during sex (AOR=1.3; 95% CI=1.1-1.6). Report of unprotected insertive anal intercourse at least once in the prior 6 months was associated with being under the influence of alcohol (AOR=1.2; 95% CI=1.0-1.5), cocaine (AOR=1.5; 95% CI=1.1-2.0) or amphetamines (AOR=1.9; 95% CI=1.4-2.6). CONCLUSIONS HIV prevention strategies for young MSM need to incorporate substance use risk reduction.