Hepatotoxicity was investigated, using plasma collected before and during treatment, in 16 human immunodeficiency virus (HIV)-hepatitis C virus (HCV)-coinfected patients who responded to highly active antiretroviral therapy (HAART), during a retrospective longitudinal study. Eleven patients experienced hepatotoxicity (i.e., a >3-fold increase in alanine aminotransferase level) while receiving HAART, including 4 patients with clinical hepatitis. Control subjects were 5 patients without hepatotoxicity. Markers of HCV-specific immune responses (HCV core-specific immunoglobulin G [IgG] antibody), T cell activation (soluble [s] CD26 dipeptidyl peptidase IV [DPP IV] enzyme activity), and inflammation (nitrate/nitrite and soluble tumor necrosis factor receptor I [sTNFRI] levels) were correlated with liver damage and immune reconstitution. All patients with hepatotoxicity had increased HCV core-specific IgG antibody and sCD26 (DPP IV) activity but did not have increased nitrate/nitrite or sTNFRI levels. Hepatotoxicity without clinical hepatitis was associated with increased CD8 T cell counts. Thus, hepatotoxicity in HIV-HCV-coinfected patients who respond to HAART is associated with increased HCV-specific immune responses and T cell activation.