A true experimental evaluation was conducted of a national volunteer service program, Teen Outreach, that was designed to prevent adolescent problem behaviors by enhancing normative processes of social development in high school students. This evaluation addressed 2 problem behaviors in adolescence—teenage pregnancy and school failure—for which experimental evidence about successful preventive programs has been largely lacking. High school students (N=695) in 25 sites nationwide were randomly assigned to either a Teen Outreach or Control group and were assessed at both program entry and at program exit 9 months later. Rates of pregnancy, school failure, and academic suspension at exit were substantially lower in the Teen Outreach group, even after accounting for student sociodemographic characteristics and entry differences between groups. Results are interpreted as suggesting the potential value both of the Teen Outreach Program specifically and also more generally of interventions that seek to prevent problem behaviors by addressing broad developmental tasks of adolescence rather than by focusing upon individual problem behaviors or micro skills.