The prevalence of attention deficit hyperactivity disorder (ADHD) with childhood onset and its relationship to course and treatment outcome of major depressive disorder (MDD) in adults was studied in 116 patients (ages 18-65) consecutively enrolled for treatment of MDD. Sixteen percent of the patient were found to meet full or subthreshold criteria for the DSM-III-R diagnosis of childhood ADHD. Twelve percent endorsed persistence of ADHD symptoms into adulthood. Depressed adults meeting criteria for childhood ADHD did not differ significantly from other depressed adults on any measures related to the chronicity or severity of the mood disorder, Axis I comorbidity, or response to acute antidepressant treatment. Our results are clinically important as they suggest that clinicians need to be aware of the possibility that a substantial proportion of patients with MDD may suffer from comorbid ADHD and that treatments need to include the targeting of possible residual ADHD symptoms in addition to those of depression.