Objective: To examine the health care costs associated with ADHD within a nationally representative sample of children. Method: 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. Results: 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. Conclusion: 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. (J. of Att. Dis. 2013; XX(X) 1-XX)