The authors present a case of coexisting obsessive-compulsive disorder (OCD) and bipolar affective disorder in which the obsessive-compulsive symptoms disappeared during episodes of mania and reappeared during periods of depression. Although patients with coexisting bipolar disorder and OCD are relatively rare, careful study of those patients may increase our understanding of the complex relationship between obsessive-compulsive symptoms and mood. Abnormalities in serotonergic neurotransmission have been postulated in both affective disorders and OCD and may provide important clues to the pathophysiology of OCD.