Objective: Data on the current costs of medical services for children with autism spectrum disorders are lacking. Our purpose for this study was to compare health care utilization and costs of children with and without autism spectrum disorders in the same health plan. Patients and Methods: Participants included all 2- to 18-year-old children with autism spectrum disorders (n = 3053) and a random sample of children without autism spectrum disorders (n = 30529) who were continuously enrolled in the Kaiser Permanente Medical Care Program in northern California between July 1, 2003, and June 30, 2004. Data on health care utilization and costs were derived from health plan administrative databases. Main Outcome Measures: Outcome measures included mean annual utilization and costs of health services per child. Results: Children with autism spectrum disorders had a higher annual mean number of total clinic (5.6 vs 2.8), pediatric (2.3 vs 1.6), and psychiatric (2.2 vs 0.3) outpatient visits. A higher percentage of children with autism spectrum disorders experienced inpatient (3% vs 1%) and outpatient (5% vs 2%) hospitalizations. Children with autism spectrum disorders were nearly 9 times more likely to use psychotherapeutic medications and twice as likely to use gastrointestinal agents than children without autism spectrum disorders. Mean annual member costs for hospitalizations ($550 vs $208), clinic visits ($1373 vs $540), and prescription medications ($724 vs $96) were more than double for children with autism spectrum disorders compared with children without autism spectrum disorders. The mean annual age- and gender-adjusted total cost per member was more than threefold higher for children with autism spectrum disorders ($2757 vs $892). Among the subgroup of children with other psychiatric conditions, total mean annual costs were 45% higher for children with autism spectrum disorders compared with children without autism spectrum disorders excess costs were largely explained by the increased use of psychotherapeutic medications. Conclusions: The utilization and costs of health care are substantially higher for children with autism spectrum disorders compared with children without autism spectrum disorders. Research is needed to evaluate the impact of improvements in the management of children with autism spectrum disorders on health care utilization and costs.