The pathogenesis of liver injury, which remains unclear in the course of human immunodeficiency virus infection, can be investigated in simian immunodeficiency virus—infected macaques, which develop an immunodeficiency disease resembling human acquired immune deficiency syndrome (AIDS). We studied the livers of 21 monkeys infected with simian immunodeficiency virus (SIVmac251) for 4 days to 39 months and detected viral antigens in Kupffer cells, macrophages, and lymphocytes in 65% of the livers tested. Virus-containing cells were present in 5 out of 9 livers tested as early as 4 days postinoculation. The number of positive cells as well as their content in viral proteins substantially increased in sinusoidal cells with the progression of the disease. Morphological features and double immuno-labeling indicated that Kupffer cells constituted the predominant cell type containing viral antigens. The presence of multinucleated giant cells displaying the ultrastructural features of resident liver macrophages was another sign of the productive infection of Kupffer cells in vivo, which was attested by the observation of budding, immature, and mature SIV particles. Kupffer cell hyperplasia and hypertrophy were evident and appeared to be related to the development of SIV infection, because a close correlation was found between antigenemia and the surface area occupied by these cells. The Kupffer cells contained apoptotic lymphocytes, indicating that resident liver macrophages could play a role in the uptake of such cells from the blood. The production of tumor necrosis factor α (TNFα) and, possibly, interferon-α by Kupffer cells, the expression of vascular adhesion molecule-1, (VCAM-1), intralobular and periportal inflammation, and the proliferation and expansion of bile duct cells were other signs of liver involvement in SIV infection.