We evaluated the association between attention deficit hyperactivity disorder (ADHD) and the age of onset of psychoactive substance use disorders (PSUD) in adults with ADHD. We hypothesized that ADHD and psychiatric comorbidity would be risk factors for early onset PSUD. We compared 120 referred adults having a clinical diagnosis of childhood-onset ADHD with 268 non-ADHD adults. All diagnoses were obtained using DSM-III-R based structured psychiatric interviews. We used group comparisons of age at onset and Cox proportional hazard models to examine the development of PSUD over time. ADHD was associated with earlier onset of PSUD independently of psychiatric comorbidity. Conduct and juvenile bipolar disorders conferred a significantly increased risk for early onset PSUD independently of ADHD. Psychiatric disorders commonly emerged before the onset of PSUD in both groups. Persistent ADHD with and without psychiatric comorbidity was associated with adolescent onset PSUD. In addition, comorbidity with conduct and juvenile bipolar disorders predicted very early onset PSUD in both ADHD and non-ADHD individuals. These findings confirm and extend previous findings documenting important associations between PSUD and psychiatric comorbidity including persistent ADHD.