Thirty-seven women with acute severe preterm placental abruption were compared with a control group of 51 women requiring preterm delivery for a medical complication of pregnancy. Histologic chorioamnionitis and funisitis were present significantly more often in patients with abruption than in control patients (41 versus 4%; P less than .0001). No patient in either group had clinical evidence of chorioamnionitis. We conclude that a significant association exists between preterm placental abruption and histologic chorioamnionitis.