A group of 480 patients, aged 19-78 with an HRSD score of at least 17 and who met DSM-III criteria for major depressive disorder were studied. Patients were given placebo for a one-week single-blind run-in period, after which sertraline was administered for eight weeks. This was followed by 44 weeks in which patients received sertraline or placebo on a double-blind, randomised basis. Patients were assessed periodically using the 17-item HRSD and the Clinical Global Impression scales. During the entire double-blind period 24 (13.0%) sertraline patients relapsed compared with 48 (45.7%) placebo patients (P less than 0.001). The protective effect of sertraline was maintained throughout the 44 weeks. The study provides evidence that sertraline prevents relapse of the index episode of depression as well as recurrence of further episodes and has few side-effects.