Obsessive-compulsive disorder is a well-defined clinical syndrome that has been difficult to treat with standard psychotherapies and medications. Data accumulated over the last decade have demonstrated that the disorder is relatively common and frequently coexists with phobia, depression, and alcohol abuse. The authors review current studies of the spectrum of obsessive-compulsive disorder and related disorders that respond to the new serotonergic antidepressants and behavioral therapy. Differential diagnosis, epidemiology and comorbidity, etiology, evaluation, and psychologic and pharmacologic treatments are discussed. Most patients with obsessive-compulsive disorder require long-term treatment with drugs, but behavioral therapy has also been used successfully. Serotonin reuptake inhibitors used in the treatment of depression have been found effective; clomipramine has produced the best results in large-scale tests. The fact that serotonin reuptake inhibitors are effective as both antidepressants and antiobsessional agents suggests common biological factors in disorders that respond to these drugs.