To evaluate the impact of new antiretroviral combinations (HAART: Highly Active Anti Retroviral Therapy) on HCV replication and liver enzyme levels, we analysed the changes in HCV viremia and aminotransferase levels in HIV and HCV co-infected patients. Moreover, to evaluate the influence of HCV infection on the efficacy of HAART, we compared the virological, immunological and biochemical response to antiretroviral combinations in anti-HIV positive subjects with or without HCV infection. We enrolled eight consecutive outpatients with HIV-HCV coinfection and with indications for HAART (Group A). For each patient in group A, we selected an anti-HIV negative patient with indications for HAART, pair-matched for age, sex, risk factor for HIV infection, presumed duration of infection, number of CD4 cells, HIV viremia and treatment schedule (Group B). A statistically significant increase in CD4 in both groups was found at 1st, 3rd and 6th month of antiretroviral therapy. A decrease in HIV-RNA in both groups was observed at 1st and 6th month of treatment. The percentage of patients with undetectable HIV-RNA at the 1st month was higher in Group B than in Group A (8/8 vs. 3/8, p = 0.025). Basal HCV-RNA viremia was very high in each case and no variations during treatment were observed. During therapy the aminotransferase levels slightly decreased in Group A and consistently increased in Group B. In Group A the differences were not significant to the statistical analysis; in Group B the aminotransferase levels at 3rd and 6th month were significantly higher than those observed at the baseline.