Objectives: To evaluate whether a behavioral intervention, which taught peer education skills, could reduce injection and sexual risk behaviors associated with primary HIV and hepatitis C virus infection (HCV) among young injection drug users (IDU). Design: We conducted a randomized controlled trial involving HIV and HCV antibodynegative IDU, aged 15–30 years, recruited in five United States cities. A six-session, small-group, cognitive behavioral, skills-building intervention in which participants were taught peer education skills (n ¼ 431) was compared with a time-equivalent attention control (n ¼ 423). Baseline visits included interviews for sociodemographic, psychosocial, and behavioral factors during the previous 3 months; HIV and HCV antibody testing; and pre/posttest counselling. Procedures were repeated 3 and 6 months postintervention. Results: The intervention produced a 29% greater decline in overall injection risk 6 months postintervention relative to the control [proportional odds ratio 0.71; 95% confidence limit (CL) 0.52, 0.97], and a 76% decrease compared with baseline. Decreases were also observed for sexual risk behaviors, but they did not differ by trial arm. Overall HCV infection incidence (18.4/100 person-years) did not differ significantly across trial arms (relative risk 1.15; 95% CL 0.72, 1.82). No HIV seroconversions were observed. Conclusion: Interventions providing information, enhancing risk-reduction skills, and motivating behavior change through peer education training can reduce injection risk behaviors, although risk elimination might be necessary to prevent HCV transmission.