The current study examines the effectiveness of a voluntary high school-based alcohol intervention by investigating one proposed mechanism of change in adolescent alcohol involvement: perception of peer use. High school students reporting lifetime drinking (N = 2055) completed fall and spring surveys that assessed demographic information, substance use, risk factors, and intervention attendance (N = 327). The motivational enhancement-based intervention used school and grade-specific normative data to facilitate change in adolescent perceptions regarding the frequency and quantity of peer alcohol use. Results indicate that intervention participants were more likely to increase the accuracy of their peer frequency estimates over the course of the year in comparison to the general student body. Furthermore, students demonstrating decreases in peer perceptions of alcohol use exhibited a greater reduction in number of binge episodes, lower maximum number of drinks consumed per episode, and average number of drinks consumed per drinking occasion (ps less than 0.05); teens with the greatest alcohol use history demonstrated the largest reductions. Although we found no significant main effect for intervention attendance, findings support the role of reduced peer drinking estimates in decreasing alcohol involvement among teen drinkers. (Contains 4 tables and 4 figures.)