Obsessive-compulsive disorder is relatively common; however, its actual incidence has only recently become clear. The neurotransmitter serotonin appears to have a central role in this disorder. Males and females are affected equally, with onset usually occurring in late adolescence. Symptoms include intrusive thoughts that lead the patient to perform repetitive rituals that interfere with daily living. Although patients are typically distressed by these thoughts and rituals, they seldom volunteer their symptoms. Successful diagnosis often requires specific questioning by the physician. Treatment is directed at symptom reduction; however, complete remission of symptoms is unusual. Pharmacologic therapy usually includes clomipramine or antidepressant treatment with selective serotonin reuptake inhibitors, but in dosage ranges higher than those typically used in the treatment of depression. Behavior therapy has also been proved effective, both alone and in conjunction with pharmacologic therapy.