The objective of this study was to use population-based data to estimate the prevalence and cost of treatment for attention-deficit hyperactivity disorder (ADHD). The North Dakota Department of Health Claims Database was used to estimate the administrative prevalence, annual cost of care per case, and total annual cost of care for subjects with ADHD compared with controls. The case population was 7745, and the mean prevalence rate was 3.9%, with a peak prevalence at 10 years of age. For children with ADHD, the annual cost of care was 649 dollars compared with that of controls at 495 dollars. Cost of care attributable to ADHD was 649 dollars - 495 dollars = 54 dollars (31%). Utilization by children with ADHD with publicly funded payers was increased 25 to 175% over that of children with privately funded coverage. In North Dakota, the annual cost of care for children with ADHD was 5.1 million dollars, 5.6% of all health care costs for children. The annual attributable cost of care was 1.79 million dollars. Thus, 1.9% of total health expenditures for children was attributable to ADHD. In the United States, the cost of care attributable to children with ADHD would be 2.15 billion dollars annually.