Evaluated were effects of a drug abuse(1) prevention program, previously shown to prevent marijuana use in adolescence, on adulthood mental health service use. Analyses were conducted on 961 6th (41%) and 7th (59%) grade participants randomly assigned to intervention or control groups at baseline in 1984. These participants were followed-up through 2003 representing 15 waves of data collection. Eighty-five percent of participants were Caucasian and 56% were female. The hypothesis was that direct program effects on early adulthood mental health service use would be mediated by program effects on high school marijuana use trajectories. Structural equation models, imputing for missing data, demonstrated that MPP (Midwestern Prevention Project) program effects on mental health were mediated by the marijuana use growth curve intercept. Findings support the role of early adolescent drug use prevention programs in impacting later mental health problems. The study's limitations are noted.