The current study reports findings from a pilot evaluation of a voluntary alcohol and marijuana intervention for young teens. Students at 2 middle schools completed 4 surveys over 2 years. During Year 2, an intervention, Project CHOICE (PC), was implemented at 1 school and was voluntarily attended by 13% of adolescents. Participants ranged from 10 to 15 years of age and were approximately 45% male, 45% White, 30% Latino, and 15% of mixed ethnic origin. Outcomes included assessments of self use and perceptions of friends' and schoolmates' past-month use of alcohol and marijuana. Analyses that compared PC participants (n=64) with a matched control sample of students (n=264) revealed that PC participants reported lower rates of alcohol use and lower perceptions of friends' marijuana use and of schoolmates' use of these substances. Random-effects growth models indicated that self use and perceptions of friends' use of alcohol and marijuana increased more sharply among control school students (n=178) relative to students from the PC school (n=270), regardless of participation. Results suggest that a brief voluntary intervention attended by a small proportion of students can impact both individual and schoolwide substance-related outcomes.