OBJECTIVES: To define and compare the prevalence, risk factors and understanding of hepatitis C transmission among Aboriginal and non-Aboriginal young offenders. DESIGN, PARTICIPANTS AND SETTING: Cross-sectional study of young offenders (aged 12-19 years; median age, 16.6 years) in custody or serving community orders with the New South Wales Department of Juvenile Justice who participated in a physical and mental health survey between March 2002 and December 2005, and who provided blood samples for analysis of biochemistry, bloodborne viruses and sexually transmitted infections. MAIN OUTCOME MEASURES: Risk factors and prevalence of hepatitis C antibody positivity. RESULTS: Of the 1042 young offenders studied, 709 provided blood samples, 179 (25%) of whom identified as Aboriginal. Aboriginal adolescents had more markers of social disadvantage and higher rates of hepatitis B (9.6% v 5.2%; P = 0.04) than non-Aboriginal young offenders. Hepatitis C rates were high in both groups (7.3% v 5.3%; P = 0.33). Risk factors for hepatitis C were the same in both groups, the most important being injecting drug use (OR, 19; P < 0.001) and prior use of heroin (OR, 15; P < 0.001). Current custodial sentence doubled the risk of hepatitis C. Knowledge of hepatitis C transmission was very poor in both groups, with over 50% not knowing how it is transmitted and fewer than 10% able to identify sharing needles as a risk. CONCLUSIONS: Hepatitis C rates are extremely high in all young offenders, and interventions to halt its spread are urgently needed. Aboriginal adolescents, who are over-represented in this population, are particularly at risk.