Objective: To test whether children with attention-deficit/hyperactivity disorder (ADHD), free of conduct disorder (CD) in childhood (mean = 8 years), have elevated risk-taking, accidents, and medical illnesses in adulthood (mean = 41 years); whether development of CD influences risk-taking during adulthood; and whether exposure to psychostimulants in childhood predicts cardiovascular disease. We hypothesized positive relationships between childhood ADHD and risky driving (in the past 5 years), risky sex (in the past year), and between risk-taking and medical conditions in adulthood; and that development of CD/antisocial personality (APD) would account for the link between ADHD and risk-taking. We report causes of death. Method: Prospective 33-year follow-up of 135 boys of white ethnicity with ADHD in childhood and without CD (probands), and 136 matched male comparison subjects without ADHD (comparison subjects; mean = 41 years), blindly interviewed by clinicians. Results: In adulthood, probands had relatively more risky driving, sexually transmitted disease, head injury, and emergency department admissions (p less than 0.05-0.01). Groups did not differ on other medical outcomes. Lifetime risk-taking was associated with negative health outcomes (p = 0.01-0.001). Development of CD/APD accounted for the relationship between ADHD and risk-taking. Probands without CD/APD did not differ from comparison subjects in lifetime risky behaviors. Psychostimulant treatment did not predict cardiac illness (p = 0.55). Probands had more deaths not related to specific medical conditions (p = 0.01). Conclusions: Overall, among children with ADHD, it is those who develop CD/APD who have elevated risky behaviors as adults. Over their lifetime, those who did not develop CD/APD did not differ from comparison subjects in risk-taking behaviors. Findings also provide support for long-term safety of early psychostimulant treatment. (Contains 3 figures and 9 tables.)