Women who have suffered one episode of postpartum-onset major depression (PPMD) comprise a high-risk group for subsequent episodes. We conducted a double-blind, randomized clinical trial to test the efficacy of nortriptyline in the prevention of recurrent PPMD. Nondepressed women who had at least one past episode of PPMD (Research Diagnostic Criteria) were recruited during pregnancy. Subjects were randomly assigned to nortriptyline or placebo. Treatment began immediately postpartum. Each subject was assessed for 20 sequential weeks with the Hamilton Rating Scale for Depression and Research Diagnostic Criteria for recurrence of major depression. No difference was found in the rate of recurrence in women treated with nortriptyline compared with those treated with placebo. Of 26 subjects who took nortriptyline preventively, 6 (0.23, 95% exact confidence interval [CI] = 0.09 to 0.44) suffered recurrences. Of 25 subjects who took placebo, 6 (0.24, 95% exact CI = 0.09 to 0.45) suffered recurrence (Fisher exact p = 1.00). Nortriptyline did not confer additional preventive efficacy beyond that of placebo. The rate of recurrence of PPMD (one fourth of women) was unacceptably high.