The placenta may play a critical role in inhibiting vertical transmission of HIV-1. Here we demonstrate that leukemia inhibitory factor (LIF) is a potent endogenous HIV-1–suppressive factor produced locally in placentae. In vitro, LIF exerted a potent, gp130-LIFRβ–dependent, HIV coreceptor–independent inhibition of HIV-1 replication with IC₅₀ values between 0.1 pg/ml and 0.7 pg/ml, depending on the HIV-1 isolate. LIF also inhibited HIV-1 in placenta and thymus tissues grown in ex vivo organ culture. The level of LIF mRNA and the incidence of LIF protein–expressing cells were significantly greater in placentae from HIV-1–infected women who did not transmit HIV-1 to their fetuses compared with women who transmitted the infection, but they were not significantly different from placentae of uninfected mothers. These findings demonstrate a novel pathway for endogenous HIV suppression that may prove to be an effective immune therapy for HIV infection.