PURPOSE: Estimate the prevalence and annual frequency of health risk indicators in young people serving community-based orders (CBOs) and custodial orders in the state of Victoria, Australia. METHODS: Cross-sectional survey of 242 young people serving CBOs and 273 serving custodial orders in Victoria in 2002-2003. Validated measures included the Composite International Diagnostic Interview for substance dependence, Short Mood and Feelings Questionnaire for depression, and Psychosis Screening Questionnaire for psychosis symptoms. Prevalence estimates were adjusted for sampling bias and age- and sex-adjusted for between-group comparisons. Prevalence estimates were applied to 2010-2011 Victorian youth justice data to estimate annual frequencies at the state level. RESULTS: The prevalence of substance dependence, poor mental health, and risky sexual behavior was high in both groups. Age- and sex-adjusted prevalence estimates were generally higher among those serving custodial orders; however, extrapolating prevalence estimates to statewide youth justice data generally resulted in higher estimated annual frequencies among CBOs. For example, the estimated prevalence of any substance dependence was 66% (95% confidence interval [CI], 60-72) in those serving custodial orders and 34% (95% CI, 26-42) in CBOs, but the estimated frequency of substance dependence in CBOs in 2010-2011 was 970 (95% CI, 750-1,180), compared with 490 (95% CI, 450-530) in those serving a custodial order. CONCLUSIONS: There is a compelling case for scaling up health services for young offenders in custody and in the community, and for routinely monitoring the health of young offenders serving custodial and community orders