This paper explores how symptoms of mental health problems influence acquired immune deficiency syndrome-related risk behaviors, and how changes in those symptoms relate to risk behaviors engaged in by young adults. Repeated interviews with 602 youths since 1984 provide a history of change in behaviors. Mental health symptoms during adolescence (alcohol/drug [r = .28]; conduct disorder [r = .27]; depression [r = .16]; suicide [r = .14]; anxiety [r = .16]; and posttraumatic stress [r = .09]) are associated with higher numbers of risk behaviors (specifically, prostitution, use of intravenous drugs, and choice of a high-risk sex partner) during young adulthood. Changes in mental health symptoms between adolescence and young adulthood are related to the number of risk behaviors engaged in by young adulthood (total number of symptoms [B = .10], alcohol/drug abuse or dependence [B = .34], depression [B = .20], suicidality [B = .35], anxiety [B = .13], and posttraumatic stress [B = .14]). Changes in symptoms of mental health problems are associated specifically with those risk behaviors that are initiated primarily in young adulthood: intravenous drug use, prostitution, and choice of risky partners. The findings show that prevention and treatment of mental health problems are important components of preventive interventions for human immunodeficiency virus infection in high-risk teens and young adults.