Descriptive norms are consistently found to predict adolescent alcohol use but less is known about the factors that predict descriptive norms. The objective of this study is to test if drinking with peers predicts later alcohol consumption and if this relationship is mediated by a change in the descriptive norms of peer alcohol use. Data are from a nationally representative cohort of high school students surveyed in the 10th and 11th grade (N = 2,162). Structural equation modeling was used to test a mediation model of the relationship between drinking with peers (T1) on later alcohol use (T2) and mediation of the relationship by descriptive norms (T2). Descriptive norms significantly mediated the relationship between drinking with peers and alcohol use for both males and females with a somewhat larger effect for males compared to females. These results support a continued focus on the development and evaluation of interventions to alter descriptive norms of alcohol use.