The frequency of attention-deficit hyperactivity disorder (ADHD) was assessed in 20 adolescents with DSM-III-R bipolar disorder hospitalized for the treatment of acute mania. Thirteen (65%) patients met DSM-III-R criteria for ADHD. Patients who met criteria for both disorders were more likely to be Caucasian and male, and to have mixed rather than pure mania. Patients with ADHD also had a higher mean total Young Mania Rating Scale score and lower serum thyroxine concentrations than patients with bipolar disorder alone. These results suggest that prior histories of ADHD or ADHD symptoms may be common in adolescents hospitalized for mania, and that patients with both disorders may have more severe manic symptoms than patients with mania alone. These findings may have important implications, especially in determining the optimal pharmacologic treatment of patients who meet criteria for both disorders, because standard treatments for ADHD and bipolar disorder have been assumed to exert opposing effects.