Forty chronic obsessive-compulsive ritualizers were randomly assigned to treatment with oral clomipramine or placebo for 8 months. During weeks 4 to 7 these two groups were each randomly split into treatment by relaxation or by exposure in vivo, and during weeks 7 to 10 all patients had exposure in vivo. Double blind assessments were made at weeks 4, 7, 10, 18, 36, 62 and 114. Results are reported to one year. Clomipramine produced significant improvement in rituals, mood and social adjustment, but only in those patients who initially had depressed mood. The clomipramine effect was maximum from weeks 10 to 18 and diminished thereafter. On stopping clomipramine patients often relapsed and improved again on restarting the drug. Relaxation produced little change. Exposure produced significant lasting improvement in rituals, but less change in mood; improvement generalized to social adjustment at follow-up. Clomipramine plus exposure had a slight additive but not interactional effect. Clomipramine enhanced compliance both with exposure and with relaxation. Clomipramine is useful for compulsive ritualizers with depressed mood, but may need continuation for over a year and combination with exposure in vivo. Exposure in vivo remains the treatment of choice for rituals without depressed mood.