Blood monocytes as well as cells of the macrophage-phagocytic system in several tissues are targets for HIV-1 in vivo and in vitro. However, the data on HIV-1 infection of liver macrophages/Kupffer cells (KCs), which make up the main pool of fixed-tissue macrophages, is controversial. We therefore studied HIV-1 infection of KCs in vivo. Blood and liver tissue was obtained from seven AIDS patients shortly after death. Liver tissue was minced before processing. Cell suspensions were further purified by density gradient centrifugation, stained with anti-CD14 (blood monocytes) or anti-macrophage 25F9 (KCs), and separated by fluorescence-activated cell sorting (FACS). These highly purified cell populations were then analyzed for HIV-1 proviral DNA by the polymerase chain reaction (PCR). By performing PCR on FACS-purified cell populations, we could show that both KCs and peripheral blood monocytes were HIV-1-infected in 3 of 7 patients. KCs harbored HIV-1 proviral DNA only if peripheral blood monocytes were infected. These data show that KCs of the liver are infected with HIV-1 in vivo.