Chorioamnionitis is a frequent finding in the placenta in second-trimester premature labor. Seventy-six archival perinatal and fetal autopsies between 15 and 28 weeks of gestation with a morphologic diagnosis of chorioamnionitis were reviewed. Of the 76 cases, 52 (68%) had inflammatory cells in the lungs, which is higher than the reported incidence of clinical infection in neonates with chorioamnionitis. In 23 cases there were peribronchial lymphocytic hyperplasia and neutrophils in the airspaces, in 8 there was a lesser degree of interstitial inflammation as well as luminal neutrophils, and in 21 (40%) there were neutrophils in the airspaces only. To determine whether the neutrophils in the airspaces in the latter were maternal or fetal in origin, the lung sections of seven male fetuses in the group were examined by in situ hybridization for the Y repeat probe pHY 2.1, together with appropriate controls. Two of the tests were technical failures. The remaining five, and the positive controls, showed positivity for pHY 2.1 in 70-86% of luminal neutrophils. We conclude that luminal neutrophils in fetal lungs in this situation are fetal in origin.