Findings show that effective early adolescent drug prevention programs that prevent marijuana use reduce participants' use of mental health services in adulthood. The analyses suggest that one mediating mechanism responsible for this relationship is weekly marijuana use upon entry to high school. These results argue strongly for the implementation of early intervention programs prior to high school, because such programs can promote positive youth development and prevent negative health behaviors that are potentially related to later mental health problems. Study data were collected as part of a long-term followup of a large drug abuse prevention trial in Kansas City, MO, in 1984 (Pentz et al., 1989). The data were obtained from a panel of students who represented 96 percent of enrolled students in eight schools. Analyses focused on marijuana use during high school and mental health services received during the last two waves of early adulthood in 1999 (age 27) and 2000-2003 (ages 28-30). The study focused on marijuana use during the high school period, because previous studies have shown that this is the developmental period in which marijuana-use peaks. The intervention component was the Midwestern Prevention Project (MPP) or Project STAR, which is a comprehensive multifaceted program for adolescent drug-use prevention. The dependent variable was mental health services received during early adulthood. Independent and mediating variables were participation in the MPP and the weekly, monthly, and lifetime use of marijuana. Since “heavy” marijuana use has been associated with poor mental health in past research, the mediating variable of primary interest was weekly marijuana use during each wave of data collection. 4 figures, 1 table, and 44 references