To examine the health care costs associated with ADHD within a nationally representative sample of children.Data were from Waves 1 to 3 (4-9 years) of the Longitudinal Study of Australian Children ( N = 4,983). ADHD was defined by previous diagnosis and a measure of ADHD symptoms (Strengths and Difficulties Questionnaire [SDQ]). Participant data were linked to administrative data on health care costs. Analyses controlled for demographic factors and internalizing and externalizing comorbidities.Costs associated with health care attendances and medications were higher for children with parent-reported ADHD at each age. Cost differences were highest at 8 to 9 years for both health care attendances and medications. Persistent symptoms were associated with higher costs ( p < .001). Excess population health care costs amounted to Aus$25 to Aus$30 million over 6 years, from 4 to 9 years of age.ADHD is associated with significant health care costs from early in life. Understanding the costs associated with ADHD is an important first step in helping to plan for service-system changes.