To study the role of infection in prematurity, we studied the demographic and obstetrical characteristics, chorioamnionic cultures, and placental histologic features of women who delivered prematurely and compared these findings with those in women who delivered at term. Microorganisms were isolated from the area between the chorion and the amnion (chorioamnion) in 23 of 38 placentas (61 percent) from women with preterm labor who delivered before 37 weeks' gestation and in 12 (21 percent) of 56 placentas from women without preterm labor who delivered at term (odds ratio, 5.6; 95 percent confidence interval, 2.1 to 15.6). The most frequent isolates from the placentas of those whose infants were delivered prematurely were Ureaplasma urealyticum (47 percent) and Gardnerella vaginalis (26 percent). The recovery of any organism from the chorioamnion was strongly associated with histologic chorioamnionitis (odds ratio, 7.2; 95 percent confidence interval, 2.7 to 19.5) and with bacterial vaginosis (odds ratio, 3.2; 95 percent confidence interval, 1.1 to 6.6). When multiple logistic regression was used to control for demographic and obstetrical variables, premature delivery was still related to the recovery of organisms from the chorioamnion (odds ratio, 3.8; 95 percent confidence interval, 1.5 to 9.9) and with chorioamnionitis (odds ratio, 5.0; 95 percent confidence interval, 1.6 to 15.3). The proportion of placentas with evidence of infection was highest among those who delivered at the lowest gestational age. We conclude that infection of the chorioamnion is strongly related to histologic chorioamnionitis and may be a cause of premature birth.