This study compared the efficacy and safety of the selective serotonin reuptake inhibitor sertraline with that of the tricyclic antidepressant clomipramine in patients with severe depression, as defined by a baseline 17-item Hamilton Depression Rating Scale (HAM-D) of at least 25. The study included 166 outpatients, randomized to double-blind treatment with sertraline (50-200 mg) or clomipramine (50-150 mg) for 8 weeks. The efficacy of both treatments was similar, 74% of patients in the sertraline group and 71% of clomipramine patients being classified as responders at the end-point, as defined by a Clinical Global Impression-Improvement (CGI-I) score of 1 or 2. Mean HAM-D scores fell from 29.8 at baseline to 12.3 at endpoint in the sertraline group, and from 29.6-12.7 in the clomipramine group. There were more withdrawals due to adverse events in the clomipramine group than in the sertraline group (17% versus 12%). Dry mouth, tremor, dizziness and constipation were all substantially more common in the clomipramine group, whereas diarrhoea/loose stools was more common in the sertraline group. Overall, sertraline was as effective as clomipramine in this group of severely depressed outpatients, and showed better tolerability.