Much information suggests that maternal reproductive tract infections, both recognized and unrecognized, account for an important and possibly preventable portion of preterm births. If such infections do mediate instances of preterm labor and premature rupture of the membranes (PROM), then associated risks of subsequent maternal and neonatal infections would be increased, even after controlling for confounding variables. To evaluate possible associations between preterm birth and maternal and neonatal infections, we conducted a retrospective study of 9642 births at the University of Colorado Health Sciences Center between July 1980 and June 1985. Clinical chorioamnionitis occurred more frequently among women delivering before term with intact membranes at the onset of labor (5.8% preterm versus 1.7% term) and among women with PROM (26.5% preterm versus 6.7% term). Among the women delivered by cesarean, the incidence of postpartum endometritis was higher in those with preterm PROM than in those with term rupture of membranes. The incidence of neonatal infection increased significantly as the gestational age of the neonates decreased (P less than .01). The rate of culture-proven neonatal infection was significantly higher following PROM (P less than .01) than after birth without PROM. Both neonatal infection and perinatal mortality were increased in association with chorioamnionitis in both preterm and term pregnancies. These consistent observations complement and support suggestions that reproductive tract infection plays a possibly preventable role in the pathogenesis of preterm birth.