OBJECTIVE: To evaluate the efficacy of an abstinence-only intervention in preventing sexual involvement in young adolescents. DESIGN: Randomized controlled trial. SETTING: Urban public schools. PARTICIPANTS: A total of 662 African American students in grades 6 and 7. INTERVENTIONS: An 8-hour abstinence-only intervention targeted reduced sexual intercourse; an 8-hour safer sex-only intervention targeted increased condom use; 8-hour and 12-hour comprehensive interventions targeted sexual intercourse and condom use; and an 8-hour health-promotion control intervention targeted health issues unrelated to sexual behavior. Participants also were randomized to receive or not receive an intervention maintenance program to extend intervention efficacy. OUTCOME MEASURES: The primary outcome was self-report of ever having sexual intercourse by the 24-month follow-up. Secondary outcomes were other sexual behaviors. RESULTS: The participants' mean age was 12.2 years; 53.5% were girls; and 84.4% were still enrolled at 24 months. Abstinence-only intervention reduced sexual initiation (risk ratio [RR], 0.67; 95% confidence interval [CI], 0.48-0.96). The model-estimated probability of ever having sexual intercourse by the 24-month follow-up was 33.5% in the abstinence-only intervention and 48.5% in the control group. Fewer abstinence-only intervention participants (20.6%) than control participants (29.0%) reported having coitus in the previous 3 months during the follow-up period (RR, 0.94; 95% CI, 0.90-0.99). Abstinence-only intervention did not affect condom use. The 8-hour (RR, 0.96; 95% CI, 0.92-1.00) and 12-hour comprehensive (RR, 0.95; 95% CI, 0.91-0.99) interventions reduced reports of having multiple partners compared with the control group. No other differences between interventions and controls were significant. CONCLUSION: Theory-based abstinence-only interventions may have an important role in preventing adolescent sexual involvement. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00640653.