The effects of human immunodeficiency virus (HIV) type 1 on the placenta and the role of the placenta in mother-to-child HIV-1 transmission are not well understood. Placentas from 78 HIV-infected and 158 HIV-uninfected women were examined as part of a prospective perinatal HIV transmission study in Bangkok. HIV-infected women were more likely than HIV-uninfected women to have chorioamnionitis (odds ratio [OR], 2.1; P = .03), placental membrane inflammation (PMI; OR, 2.7; P = .02), and deciduitis (OR, 2.3; P = .03) and less likely to have villitis (OR, 0.3; P = .02). However, among HIV-infected women, fewer women who transmitted infection to their child had chorioamnionitis (relative risk [RR], 0.2; P = .03), funisitis (RR, 0.4; P = .1), or PMI (RR undefined; P = .03). These findings suggest that, in this population, HIV-infected women are at increased risk for placental membrane inflammatory lesions, but that placental inflammatory lesions are not associated with increased perinatal HIV transmission.