CONTEXT Prospective studies of childhood attention-deficit/hyperactivity disorder (ADHD) have not extended beyond early adulthood. OBJECTIVE To examine whether children diagnosed as having ADHD at a mean age of 8 years (probands) have worse educational, occupational, economic, social, and marital outcomes and higher rates of ongoing ADHD, antisocial personality disorder (ASPD), substance use disorders (SUDs), adult-onset psychiatric disorders, psychiatric hospitalizations, and incarcerations than non-ADHD comparison participants at a mean age of 41 years. DESIGN Prospective, 33-year follow-up study, with masked clinical assessments. SETTING Research clinic. PARTICIPANTS A total of 135 white men with ADHD in childhood, free of conduct disorder, and 136 men without childhood ADHD (65.2% and 76.4% of original cohort, respectively). MAIN OUTCOME MEASURES Occupational, economic, and educational attainment; marital history; occupational and social functioning; ongoing and lifetime psychiatric disorders; psychiatric hospitalizations; and incarcerations. RESULTS Probands had significantly worse educational, occupational, economic, and social outcomes; more divorces; and higher rates of ongoing ADHD (22.2% vs 5.1%, P < .001), ASPD (16.3% vs 0%, P < .001), and SUDs (14.1% vs 5.1%, P = .01) but not more mood or anxiety disorders (P = .36 and .33) than did comparison participants. Ongoing ADHD was weakly related to ongoing SUDs (ϕ = 0.19, P = .04), as well as ASPD with SUDs (ϕ = 0.20, P = .04). During their lifetime, probands had significantly more ASPD and SUDs but not mood or anxiety disorders and more psychiatric hospitalizations and incarcerations than comparison participants. Relative to comparisons, psychiatric disorders with onsets at 21 years or older were not significantly elevated in probands. Probands without ongoing psychiatric disorders had worse social, but not occupational, functioning. CONCLUSIONS The multiple disadvantages predicted by childhood ADHD well into adulthood began in adolescence, without increased onsets of new disorders after 20 years of age. Findings highlight the importance of extended monitoring and treatment of children with ADHD.