At least 50% of patients with manic depressive disorder (MDD) treated successfully with electroconvulsive therapy (ECT) will experience a relapse within the first year of follow-up. Sleep disturbances are very common in MDD and may constitute forerunners of relapse. In this study we tested the hypothesis that melatonin, a sleep-promoting hormone, would decrease the 3-month relapse rate after successful ECT. We included in the study patients with MDD successfully treated with ECT (post-ECT Hamilton Rating Scale for Depression [HRSD] less than or equal to 10). Patients were blindly randomized to two groups, one receiving fluoxetine + placebo and one receiving fluoxetine + melatonin. Assessments (HRSD, Brief Psychiatric Rating Scale, Global Assessment of Function Scale, Global Depression Scale. Pittsburgh Sleep Quality Index, Mini-Mental State Exam. and pill count) were performed for 12 weeks after ECT. Ten of the 35 patients (28.5%) relapsed during the follow-up period. Relapse rates were similar in both groups of patients. Sleep reports were not improved by melatonin. Patients who achieved a higher functional state post-ECT relapsed less often. We conclude that the addition of melatonin to on-going fluoxetine treatment did not have a beneficial effect either on the 3-month outcome post-ECT or on the sleep reports of these patients.