The purpose of the study was to evaluate the impact of the Going Places Program and mediation of treatment effects. Seven middle schools were randomized to intervention or comparison conditions and students (n = 1,320) in two successive cohorts provided five waves of data from sixth through eighth grade. The Going Places Program included classroom curriculum, parent education, and school environment components. Latent growth curve analyses demonstrated significant treatment group effects on outcome expectancies, friends who smoke, and smoking. Friends who smoke mediated the program effect on adolescents' smoking progression. The protective effect of the Going Places Program on smoking progression was due in part to the prevention of increases in friends who smoke. (Contains 1 table and 4 figures.)