OBJECTIVE To know the efficiency and tolerance of INF therapy for chronic virus C hepatitis (HCV) in HIV infected patients compared with non infected patients. METHODS INF-alpha was administered to 39 patients with chronic hepatitis C virus infection criteria. In 17 cases (43.5%) there was coinfection with HIV. Histologic data were available from 30 patients (75%) and also of viral load during therapy (Amplicor HCV, Roche Diagnostics) from 8 patients. We determined the response at the end of the first two months of therapy (ER), at the end of therapy (FR) and after discontinuation (DR) when the transaminase level was normalized and viral RNA was not detected in cases when it was measured. The response rates to INF were compared between HIV-positive and HIV-negative patients and the secondary effects observed evaluated, as well as tolerance and severity, with a particular emphasis on the CD4 lymphocyte level among HIV-positive patients. RESULTS An ER was obtained in nine HIV-positive patients (52.9%) and thirteen HIV-negative patients (59%); an FR in eight HIV-positive patients (47%) and eleven HIV-negative patients (50%), and DR in two HIV-positive patients (13.3%) and four HIV-negative patients (28%); although a lower rate of DR was observed among HIV-positive patients, these differences were not significant. The disappearance of HCV ARN at the end of therapy was similar for both groups of patients in whom it was measured: five HIV-positive patients (62.5%) and twelve HIV-negative patients (63.1%). We must consider that HIV-positive patients had a higher number of poor response predictors to INF. Secondary reactions were observed in a higher number of HIV-negative patients (81.8% versus 40.9%) and the level of CD4 lymphocytes was markedly reduced during and after therapy in three patients. CONCLUSIONS INF therapy in chronic hepatitis C virus infection in HIV-positive patients initially has a similar efficiency to that observed in HIV-negative patients, although perhaps the maintained response rate is lower. A higher number of secondary reactions among HIV-positive patients was not observed, although possible reductions in CD4 levels must be considered among these patients. The use of INF in these patients --if properly selected--is therefore not contraindicated.