Objective: Histologic chorioamnionitis has been associated with preterm birth and low birthweight. Our goal was to evaluate the relationship between polymorphonuclear cell and mononuclear cell placental infiltrations and adverse infant outcomes. Study design: Data from women who were enrolled in a trial of antibiotics for prevention of mother-to-child transmission of HIV-1 and of preterm birth were analyzed. Women who had HIV and women who did not have HIV were assigned randomly to either metronidazole 250 mg and erythromycin 250 mg at 20 to 24 weeks of gestation 3 times daily for 7 days and metronidazole 250 mg and ampicillin 500 mg every 4 hours during labor or identical placebos. Women with HIV were offered nevirapine at delivery for the prevention of mother-to-child transmission. At delivery, various placental sites were evaluated for polymorphonuclear cell and mononuclear cell infiltration. Results: Polymorphonuclear and mononuclear cell infiltrations were common in the decidua (18% and 43%, respectively) and chorioamnion (28% and 34%, respectively). Gestational age and birthweight were lower among women with polymorphonuclear cell infiltrations, with these relationships generally stronger at earlier gestational age and birthweight. Mononuclear infiltrations were not associated with adverse outcomes. Neither polymorphonuclear cell nor mononuclear placental infiltrations were associated with mother-to-child transmission of HIV. Antibiotic use was not associated with reduced polymorphonuclear or mononuclear cell infiltrations. Conclusion: Polymorphonuclear cell infiltrations were associated with preterm birth and decreased birthweight and gestational age. Antibiotic use was not associated with reductions in polymorphonuclear or mononuclear cell infiltrations. In this nevirapine-treated population, neither polymorphonuclear nor mononuclear cell infiltration was associated with the mother-to-child transmission of HIV. © 2006 Mosby, Inc. All rights reserved.