BACKGROUND: Despite data describing the overlap of attention deficit hyperactivity disorder (ADHD) and bipolar disorder (BPD) in youth, little is known about adults with these co-occurring disorders. We now evaluate the clinical characteristics of referred adults with (n = 24) and without BPD (n = 27). METHODS: Referred adults to clinical trials of ADHD were evaluated by psychiatric evaluation using DSM-IV criteria. Structured psychiatric interviews were used to systematically assess adult and childhood disorders. RESULTS: The vast majority of patients with ADHD plus BPD had bipolar II disorder (88%). Adults with ADHD plus BPD had higher rates of the combined subtype of ADHD compared to ADHD without BPD (chi(2) = 8.7, p =.003), a greater number of DSM-IV ADHD symptoms (14.8 +/- 2.9 and 11.4 +/- 4.0; t = -3.4, p <.01), more attentional symptoms of ADHD (8.1 +/- 1.4 and 6.8 +/- 2.1; t = -2.5, p <.02; trend), poorer global functioning (47 +/- 5.9 and 52 +/- 7.4, t = 2.6, p <.02; trend), and additional comorbid psychiatric disorders (3.7 +/- 2.5 and 2.0 +/- 1.9; t = -2.9, p <.01). CONCLUSIONS: These results suggest that adults with ADHD plus BPD have prototypic symptoms of both disorders, suggesting that both disorders are present and are distinguishable clinically.