Thirty-eight depressed patients who were treated with ECT were randomly assigned to receive lithium therapy or identical-looking placebo tablets for one year after clinical recovery in a double-blind trial. The patients who received placebo tablets spent an average of 7.8 weeks with an episode of depression (either as in-patients or day-patients) during the year. In comparison, patients who received lithium spent on average 1.7 weeks with an episode (P less than 0.02). The trial confirms the high rate of relapses after ECT and suggests that lithium considerably reduces this morbidity. It is suggested that ECT without continuation therapy is not a satisfactory treatment of depressive illness.