Examined a program designed to prevent adolescent pregnancy, school failure, and dropout using a process model of evaluation to assess with which groups of participants and under what conditions the program was most effective. Students in the Teen Outreach Program of the Association of Junior Leagues and matched comparison students in 35 schools nationwide participated. Sites that highly utilized a volunteer service component, and sites that primarily served older students reported lower levels of student problem behaviors at program exit, after controlling for problem behaviors at entry. These findings occurred only for program youths and not for comparison youths. The connection of volunteer service to reductions in adolescent problem behaviors is interpreted in terms of helper-therapy and empowerment theories. Limitations of the analytic strategy used in this study, as well as techniques for addressing the limitations, are also discussed.