Service use patterns and costs of youth diagnosed with attention-deficit/hyperactivity disorder (ADHD) and comorbid conduct disorder (CD) were assessed across adolescence (ages 12 through 17). Featured service sectors include mental health, school services, and the juvenile justice system. Data are provided by three cohorts from the Fast Track evaluation and are based on parent report. Diagnostic groups are identified through a structured assessment. Results show that public costs for youth with ADHD exceed $40,000 per child on average over a 6-year period, more than doubling service expenditures for a non-ADHD group. Public costs for children with comorbid ADHD and CD double the costs of those with ADHD alone. Varying patterns by service sector, diagnosis, and across time indicate different needs for youth with different conditions and at different ages and can provide important information for prevention and treatment researchers.