Liver disease secondary to hepatitis C virus (HCV) infection is a rising cause of morbidity and mortality among individuals who have been infected parenterally with human immunodeficiency virus (HIV) such as injection drug users, hemophiliacs, and transfused patients. We analyzed both the efficacy of interferon (IFN) alpha therapy in these patients and the predictors of response to this agent. A total of 119 patients with chronic hepatitis C (90 of whom were infected with HIV and 29 of whom were not) were included in a multicenter, prospective, open, nonrandomized observational study. IFN-alpha was given subcutaneously in a dosage of 5 million units three times a week during a 3-month period; those patients who responded received a dose of 3 million units given subcutaneously three times a week for an additional 9 months. One hundred seven patients completed the study; the level of aminotransferases returned to normal and sera became negative (complete response) for HCV RNA in 26 (32.5%) of 80 HIV-infected patients and 10 (37.0%) of 27 non-HIV-infected patients (P = .666) after completion of the treatment. Two variables were independently associated with a response in HIV-infected patients: a CD4+ T lymphocyte count of > 500 x 10(6)/L and a baseline HCV viremia level of < 10(7) copies/mL. In the 12 months following treatment, relapses occurred in 30.8% of the HIV-infected patients and 12.5% of non-HIV-infected patients (P = .403).