Chorioamnion infection is associated with histologic chorioamnionitis and prematurity, but the specific chorioamnion microorganisms associated with histologic chorioamnionitis, prematurity, and poor neonatal outcome have not been identified. Bacteria were recovered from the chorioamnion cultures of 32% of 112 placentas delivered at less than or equal to 34 weeks' gestation and from 19% of 156 placentas delivered at greater than 34 weeks' gestation (odds ratio 2.1; 95% confidence interval 1.1 to 3.8). Chorioamnion bacteria most highly related to both prematurity and histologic chorioamnionitis were group B Streptococcus and Fusobacterium species. Chorioamnion infection with Peptostreptococcus was significantly related only to preterm delivery, and infection with Escherichia coli, Bacteroides, and Ureaplasma were significantly related to histologic chorioamnionitis. Among preterm infants, isolation of bacteria from the chorioamnion was related to an increased risk of neonatal death (rate ratio 3.8; 95% confidence interval 1.4 to 11.6). Bacterial infection of the chorioamnion is related to preterm birth, histologic chorioamnionitis, and neonatal death.