The potential impact of the hepatitis C virus (HCV) on clinical, immunological and virological responses to initial highly active antiretroviral therapy (HAART) of patients infected with human immunodeficiency virus (HIV) is important to evaluate due to the high prevalence of HIV-HCV coinfection. A historical cohort study was conducted among 824 HIV-infected patients starting HAART at a public referral service in Belo Horizonte, Brazil, to assess the impact of HCV seropositivity on appearance of a new AIDS-defining opportunistic illness, AIDS-related death, suppression of viral load, and an increase in CD4-cell count. A total of 76 patients (9.2%) had a positive HCV test, 26 of whom (34.2%) had a history of intravenous drug use. In multivariate analysis, HCV seropositivity was associated with a smaller CD4-cell recovery (RH=0.68; 95% CI [0.49-0.92], but not with progression to a new AIDS-defining opportunistic illness or to AIDS-related death (RH=1.08; 95% CI [0.66-1.77]), nor to suppression of HIV-1 viral load (RH=0.81; 95% CI [0.56-1.17]) after starting HAART. These results indicate that although associated with a blunted CD4-cell recovery, HCV coinfection did not affect the morbidity or mortality related to AIDS or the virological response to initial HAART.