This paper analyzes the impact of increases in the minimum drinking age on the prevalence of alcohol and marijuana use among high school seniors. The empirical analysis is based on a large sample of students from 43 states over the years 1980-1989. We find that increases in the legal minimum drinking age did slightly reduce the prevalence of alcohol consumption. We also find, however, that increased legal minimum drinking ages had the unintended consequence of slightly increasing the prevalence of marijuana consumption. Estimates from a structural model suggest that this unintended consequence is attributable to standard substitution effects.