Context: Postpartum psychosis occurs in 1 to 2 cases per 1000 live births. Most studies have not distinguished postpartum psychosis from bipolar disorder or the proportion of the incidence attributable to prepregnancy psychiatric morbidity. Objective: To determine the incidence of postpartum psychosis and bipolar disorder attributable to previous psychiatric hospitalization. Design: Population-based study using linked registry data to determine postpartum onset of psychotic and bipolar episodes within 90 days after the first birth, by women with and without prepregnancy or prenatal psychiatric hospitalization. We assessed the type, number, and recency of previous hospitalizations on the incidence of hospitalization for postpartum psychotic and bipolar episodes. Setting: Nationwide Swedish Hospital Discharge and Medical Birth registers. Patients: Swedish women delivering a first live infant between January 1, 1987, and December 31, 2001. Main Outcome Measures: Postpartum hospitalization for psychosis or bipolar disorder. Results: The cumulative incidences for postpartum psychotic and bipolar episodes (adjusted for age at first birth) were 0.07% and 0.03%, respectively. The incidence of psychiatric hospitalizations for postpartum psychotic or bipolar episodes among women without previous psychiatric hospitalizations was 0.04% and 0.01% of first births, respectively; for women with any psychiatric hospitalization before delivery, the incidence was 9.24% and 4.48%, respectively. For postpartum psychotic and bipolar episodes, the risk increased significantly with the recency of prepregnancy hospitalizations, number of previous hospitalizations, and length of most recent hospitalization. More than 40% of women hospitalized during the prenatal period for a bipolar or a psychotic condition were hospitalized again during the postpartum period. Approximately 90% of all postpartum psychotic and bipolar episodes occurred within the first 4 weeks after delivery. Conclusions: Almost 10% of women hospitalized for psychiatric morbidity before delivery develop postpartum psychosis after their first birth. This underscores the need for obstetricians to assess history of psychiatric symptoms and, with pediatric and psychiatric colleagues, to optimize the treatment of mothers with psychiatric diagnoses through childbirth.