This article reviews the literature concerning obsessive-compulsive disorder (OCD) and its treatment with drugs. We survey the differential diagnosis of OCD and the problems involved in evaluating its severity in a research setting. Pharmacological treatment of OCD has usually involved antidepressants, especially clomipramine, with equivocal but nevertheless encouraging success. Anxiolytics and neuroleptics have also been tried, but the results are inconsistent. We discuss the implications of these findings for treatment of patients with OCD and for future research.