The entire early adolescent population of the 15 communities that constitute the Kansas City (Kansas and Missouri) metropolitan area has participated in a community-based program for prevention of drug abuse since September 1984. The Kansas City area is the first of two major metropolitan sites being evaluated in the Midwestern Prevention Project, a longitudinal trial for primary prevention of cigarette, alcohol, and marijuana use in adolescents. The project includes mass media programming, a school-based educational program for youths, parent education and organization, community organization, and health policy components that are introduced sequentially into communities during a 6-year period. Effects of the program are determined through annual assessments of adolescent drug use in schools that are assigned to immediate intervention or delayed intervention control conditions. In the first 2 years of the project, 22,500 sixth- and seventh-grade adolescents received the school-based educational program component, with parental involvement in homework and mass media coverage. Analyses of 42 schools indicate that the prevalence rates of use for all three drugs are significantly lower at 1-year follow-up in the intervention condition relative to the delayed intervention condition, with or without controlling for race, grade, socioeconomic status, and urbanicity (17% vs 24% for cigarette smoking, 11% vs 16% for alcohol use, and 7% vs 10% for marijuana use in the last month), and the net increase in drug use prevalence among intervention schools is half that of delayed intervention schools.