OBJECTIVE Factors mediating changes in sexual behaviors that increase the risk of human immunodeficiency virus (HIV) infection were monitored in a group of gay and bisexual male adolescents. METHODS One hundred thirty-six males aged 14 to 19 years (Hispanic, 51%; African-American, 31%) were recruited from one gay-identified agency, were assessed at four points over a 1-year period, and participated in HIV preventive intervention sessions. RESULTS Significant reductions occurred in the number of unprotected same-sex anal and oral acts. Those with less risk in their previous sexual history, those who did not engage in commercial sex, and those who attended more HIV intervention sessions were more likely to reduce their sexual risk. The impact of sessions varied significantly by race/ethnicity: African-American youths reduced their risk acts most dramatically. Abstinence was consistently and significantly more likely among younger youths and those who had been abstinent before enrollment. The youths significantly reduced the number of sexual partners following the intervention; this reduction in partners was maintained through the 12-month follow-up and was greatest among youths with no involvement in commercial sexual activity (prostitution). CONCLUSIONS The efficacy of HIV prevention programs must be empirically evaluated.