The purpose of this study was to determine the incidence of clinically significant depression occurring between 1 and 4 months postpartum and to investigate whether somatic complaints, subsyndromal depressive symptoms, or birth-related concerns among non-depressed women at 1 month were predictive of postpartum depression. This is a prospective cohort study of 465 women from the Wisconsin Maternity Leave and Health Project (WMLHP). Women who were not depressed at 1 month postpartum were reassessed 3 months later for depression occurring at any time in the interval between 1 and 4 months postpartum. Depression was defined as either meeting the criteria for major depression on the National Institute of Mental Health (NIMH) Diagnostic interview Schedule (DIS) or scoring above 15 on the Center for Epidemiologic Studies Depression Scale (CES-D). Physical symptoms were assessed by an adapted Health Responses Scale. Other measures were developed specifically for the WMLHP. Of 465 women, 27 (5.8%) became clinically depressed between 1 and 4 months postpartum. In a logistic regression analysis, four variables (maternal age, depression during pregnancy, thoughts of death and dying at 1 month postpartum, and difficulty falling asleep at 1 month postpartum) were predictive of depression at 4 months postpartum. Breast-feeding, mode of delivery, family income, parity and mother's education did not predict depression. The existence of subsyndromal depressive symptoms, particularly thoughts of death and dying, may represent a prodromal phase of depression and should alert clinicians to the possibility of future postpartum depression. Women with a history of depression during pregnancy should be monitored for signs of postpartum depression for a minimum of 4 months. Obstetricians are in a unique position during the postpartum check-up to screen women for these predictors of future postpartum depression and possibly to avert the development of a clinically significant depressive episode.