HIV-negative, inner-city adolescents with HIV-infected parents are considered to be at high risk for acquiring HIV themselves. Using a modified theory of health behavior, this study examined the effects of maternal HIV infection and psychosocial variables on the onset of sexual and drug risk behavior in 144 HIV-negative adolescents with and without HIV-positive mothers. Adolescents and their mothers were interviewed when the youths were 10-14 years old and again when they were 13-19 years old. By follow-up, 42% of youths reported the onset of vaginal sex (vs 5% at baseline). Marijuana and alcohol use increased from 6 and 38%, respectively, at baseline to 25 and 60% at follow-up. Among those reporting risk behaviors, 40-50% reported onset prior to 14 years. Youth and family psychosocial variables, but not maternal HIV status, were associated with risk behaviour outcomes.