The pathology of term placentas from seropositive human immunodeficiency virus (HIV)-infected and seronegative women was investigated by routine histologic, immunocytochemical, and in situ hybridization techniques. Placentas were evaluated for evidence of villitis, chorioamnionitis, and funisitis. Membranes, trophoblast, and decidua were also examined by immunohistochemistry using monoclonal HIV p24 antibody. Twenty placentas were evaluated by combined immunochemical and in situ hybridization techniques, using a 35S-labeled RNA probe complementary to the 3' long terminal repeat and envelope region of HIV-1. HIV-seropositive placentas did not show significant villitis; however, the incidence of chorioamnionitis increased (P less than .01). HIV antigens and nucleic acids were identified in the trophoblast of 10% of the placentas that also showed chorionitis. Term HIV-positive placentas may show histologic changes that may or may not be directly related to the virus. Analysis of tissues from earlier gestational placentas may prove more informative in clarifying the mechanism of maternal-fetal HIV transmission.