Patients who have suffered a depressive illness frequently relapse when treatment is stopped. A controlled study of continued medication after clinical recovery was carried out. Of 92 patients studied, 22% of those who received amitriptyline or imipramine relapsed during 6 months of observation, compared with 50% of those who received placebo. Patients who experienced persistent residual symptoms derived more benefit from the active drug during continuation treatment than those who made a complete recovery from their illness.