Rhesus monkeys infected with simian immunodeficiency virus (SIV) develop a syndrome very similar to patients with acquired immune deficiency (AIDS), including liver disease. This prospective study was undertaken to define the pathology, course, and pathogenesis of liver disease in 20 rhesus monkeys (Macaca mulatta) after intravenous inoculation with the standardized isolate SIV/DeltaB670. Tissue samples from liver and gallbladder between 2 and 24 weeks after inoculation were examined histologically and immunohistochemically for SIV gag protein p26, and by in situ hybridization with an SIV riboprobe. Histologically there was infiltration of portal tracts and around hepatic veins and venules by mononuclear inflammatory cells, focal bile duct damage, proliferation of bile ductules, and focal lobular inflammation as early as 2 weeks after infection. The severity and extent of these lesions were graded semiquantitatively and showed that bile duct damage and hepatic venulitis were the most significant changes. Simian immunodeficiency virus gag protein p26 and SIV RNA were detected in scattered mononuclear cells in portal tracts and sinusoids, but not in hepatocytes or bile duct epithelial cells. The data indicate that the liver is involved early during the course of SIV infection, followed by persistent changes until the terminal stage of the disease. Our findings suggest that the liver damage in SIV-infected rhesus monkeys is similar to the changes observed previously in AIDS patients.