Hepatitis C virus (HCV) has emerged as a major pathogen among patients with human immunodeficiency virus (HIV). Morbidity and mortality were compared among 263 patients with HIV alone, 166 patients with HIV and HCV, and 60 patients with HCV alone (mean duration of follow-up, 2 years and 10 months). No differences in HIV loads and CD4 cells counts were observed between the HIV and HIV/HCV groups. Alanine aminotransferase levels were higher (52 U/L versus 35 U/L; P<.05) and albumin levels were lower (3.5 g/dL versus 3.8 g/dL; P <.02) among coinfected patients than they were among patients with HIV alone. Liver decompensation developed in 10% of patients with HIV/HCV coinfection. In contrast, no liver-related deaths or decompensation occurred in patients without coinfection (P<.05). Of the patients with HIV alone, 7% died, compared with 11% of the coinfected patients (P<.02); 47% of the deaths in the latter group were due to liver-related causes. In summary, HCV infection causes increased morbidity and mortality in patients with HIV infection.