Clinical pictures and certain sociodemographic factors were studied retrospectively in all women in the county of Stockholm who had a baby during 1976-77 and also had been admitted to a psychiatric department after the 20th week of pregnancy or during the first postpartum year. The sample was classified according to the Research Diagnostic Criteria. A significant rise in the incidence of mental illness was found within the first 3 months postpartum and particularly so within 1 month postpartum as compared with during pregnancy and the following 9 months. This peak in incidence was due to a rise in the number of women classified as Unspecified Functional Psychosis. The nosology of postpartum psychosis is discussed. An increased frequency of neurotic or minor psychiatric disorders was noted during pregnancy as compared with the first 3 months postpartum and the following 9 months. No associations were found between the sociodemographic factors considered and the time of illness onset, although marital discord tended to be more frequent in women with onset of illness during pregnancy.