Microscopic features of placentas from 539 consecutive preterm deliveries and 214 term deliveries were compared. The presence of either umbilical or chorionic vasculitis was identified in 38% of the cases at 22 to 28 weeks' gestation, in 32% of the cases at 29 to 32 weeks' gestation, in 13% of the cases at 33 to 36 weeks' gestation, and in 10% of the cases at term (p less than 0.0001). Decidual vascular abnormality was present in 70% of the cases at 22 to 28 weeks' gestation, in 35% of the cases at 29 to 32 weeks, in 29% of the cases at 33 to 36 weeks, and in 15% of the cases at term (p less than 0.0001). Chronic villitis was significantly more frequent in preterm deliveries without umbilical vasculitis than in those cases with umbilical vasculitis (17% vs 8%, p less than 0.05). Our data indicate that the placental lesions of umbilical-chorionic vasculitis, decidual vascular abnormality, and chronic villitis are related to preterm birth. Umbilical-chorionic vasculitis reflects acute ascending bacterial infection. Decidual vascular abnormality has been associated with maternal autoimmune or alloimmune disorders. Chronic villitis may indicate either congenital viral infection or maternal-fetal immunopathologic conditions. Both decidual vascular abnormality and chronic villitis may reflect the activation of inflammatory mechanisms capable of leading to preterm delivery.