Depression is a serious mental health problem, affecting more young people than ever before. This research reports on the short-term effectiveness of an Optimism and Lifeskills Program for preventing depression in preadolescents. A randomised, controlled trial was conducted with students about to make their transition to high school, in a private girls school. Self-report questionnaires were used to assess the program's effect on depressive and lonely symptoms, attributional style and self-worth, in a sample of 47 grade 7 girls at posttest and 6-month follow-up. Results showed fewer depressive symptoms and more positive self-worth in the intervention group compared to the control group at 6-month follow-up. This research indicates that depression prevention programs can be effective in the short-term and can provide valuable skills to young people making their transition to high school. Implementation issues and suggestions for further research on primary prevention for mental health in schools are discussed.