This population-based study, which enrolled 105,909 women having singleton deliveries in the years 1988 to 1999, was done to assess the obstetric features and perinatal outcome of pregnancies accompanied by uterine leiomyomas. All participants were seen at 24 or more weeks gestation and received perinatal care. Leiomyomas were present in 690 women, 0.65% of the study group. These women were older than the others and were more often nulliparous. Their infants were of lesser gestational age when born and had lower birth weights. Rates of hypertensive disorders, gestational and pregestational diabetes, fertility treatment, intrauterine growth retardation, and premature rupture of membranes all were greater in women with leiomyomas than in those without. In addition, these women more often reported previous recurrent abortions, previous cesarean delivery, hydramnios, and oligohydramnios. Complications, including placental abruption, placenta previa, malpresentation, failure of labor to progress, postpartum bleeding, and retained placenta, were more prevalent in women with leiomyomas. In addition, they more often received packed cell transfusions during pregnancy. Perinatal mortality was increased compared with women without uterine leiomyomas (odds ratio [OR], 1.8). On multivariate analysis, nulliparity, chronic hypertension, hydramnios, diabetes, and older maternal age were significantly associated with the presence of leiomyomas. After adjusting for maternal age, parity, gestational age, and malpresentation, women with leiomyomas had higher rates of cesarean section (OR, 6.7), placental abruption (OR, 2.6), and delivery before 36 weeks gestation (OR, 1.4). The OR for perinatal mortality was reduced to 1.4 in multivariable analysis. Women with uterine leiomyomas should be aware of their increased risk of adverse pregnancy outcomes and of the need for appropriate intrapartum management.