OBJECTIVES: To estimate the excess costs for children in the years surrounding initial diagnosis of attention-deficit/hyperactivity disorder (ADHD) and to estimate differences in treatment costs by ethnicity. DESIGN: We identified children diagnosed with ADHD and estimated their health service costs in the 2 years before and 2 years after initial diagnosis of ADHD. Costs were compared with those for children without ADHD. We adjusted for age, sex, ethnicity, pharmacy co-pay, estimated family income, coexisting mental health disorders, and chronic medical conditions. SETTING: Nonprofit, integrated health care delivery system in northern California from January 1, 1996, to December 31, 2004. PARTICIPANTS: Children aged 2 to 10 years with (n = 3122) and without (n = 15 899) ADHD. Main Exposure Attention-deficit/hyperactivity disorder. MAIN OUTCOME MEASURES: Health care costs and use in the years before and after initial ADHD diagnosis as well as costs of ADHD-related services. RESULTS: Compared with children without ADHD, children with ADHD had mean costs that were $488 more in the second year before their ADHD diagnosis, $678 more in the year before their diagnosis, $1328 more in the year after their diagnosis, and $1040 more in the second year after their diagnosis. Asian Americans diagnosed with ADHD had lower total ADHD-related mean costs per year than white Americans diagnosed with ADHD ($221 lower), and Asian Americans, African Americans, and Hispanic Americans all had lower ADHD-related pharmacy mean costs than white Americans ($95, $63, and $77 lower, respectively). CONCLUSIONS: Children with ADHD use significantly more health services before and after their diagnosis than children without ADHD. Among children diagnosed with ADHD, nonwhite Americans (especially Asian Americans) use fewer ADHD-related services than white Americans.