OBJECTIVE: The purpose of this study was to examine the rate of attention deficit hyperactivity disorder in adolescents with bipolar disorder and to explore the potential effects of comorbid attention deficit hyperactivity disorder on the phenomenology of adolescent bipolar disorder. METHOD: The authors assessed the rate of attention deficit hyperactivity disorder for adolescents with bipolar disorder who were hospitalized for treatment of acute mania or hypomania. RESULTS: Eight (57%) of 14 adolescent bipolar patients also met DSM-III- R criteria for attention deficit hyperactivity disorder. Patients with both disorders were more likely to be male and Caucasian and to have mixed rather than manic bipolar disorder. Patients with attention deficit hyperactivity disorder had a higher mean total score on the Young Mania Rating Scale than patients with bipolar disorder alone. CONCLUSIONS: Although preliminary, these findings may have important implications regarding the potential relationship between bipolar disorder and attention deficit hyperactivity disorder.