Premature separation of the normally implanted placenta is a serious complication of pregnancy and a leading cause of maternal and perinatal morbidity and mortality. Using data from the National Hospital Discharge Survey, we estimated rates of abruptio placentae in the United States for the years 1979-1987 and examined the association of this condition with several demographic risk factors and coexisting obstetric conditions. In 1987, the national rate was 11.5 cases per 1000 deliveries. The rate of abruptio placentae increased significantly between the years 1979-1987 among women of all racial groups. The increase in the rate of placental abruption occurred mainly among women under the age of 25, unmarried women, and women on Medicaid compared with those who had private insurance. Women with placental abruption were 54 times more likely to have coagulopathies and 11 times more likely to have stillbirths than those without placental abruption. Twin gestations, preterm premature rupture of membranes, chorioamnionitis, chronic hypertension, and preeclampsia/eclampsia were also associated with placental abruption. Although the cause for the increase in the incidence of abruptio placentae is not known, most of the increase occurred among women likely to be financially and socially disadvantaged.