Symptoms of DSM-IV attention-deficit hyperactivity disorder (ADHD) were determined in patients entering methadone maintenance treatment. The relationship of ADHD to psychiatric and substance abuse comorbidity, attention testing, and treatment outcome was analyzed; 19% of patients had a history of ADHD, and 88% of these had current symptoms. Continuous Performance Testing indicated poorer attention in patients with ADHD. The only substance use disorder more common in the ADHD group was clonidine. There was significantly more current axis I, dysthymic disorder, anxiety disorder (including social phobia), and antisocial personality disorder in the ADHD patients. There was no difference between groups at the 1-year follow-up for illicit drug use, treatment retention, or treatment performance. The ADHD diagnosis did not convey significant prognostic implications for methadone maintenance treatment. A strong psychiatric assessment and treatment focus in the treatment program may help to explain the good treatment outcome.