OBJECTIVE: To assess hepatitis C virus (HCV) antibody prevalence in the EuroSIDA cohort, along with survival, human immunodeficiency virus (HIV)-1 disease progression, virologic response (plasma HIV-1 RNA load of < 500 copies/mL), and CD4 cell count recovery by HCV serostatus in patients initiating highly active antiretroviral therapy (HAART). RESULTS: HCV serostatus at or before enrollment was available for 5957 patients; 1960 (33%) and 3997 (67%) were HCV seropositive and seronegative, respectively. No association between an increased incidence of acquired immunodeficiency syndrome-defining illnesses or death and HCV serostatus was seen after adjustment for other prognostic risk factors known at baseline (adjusted incidence rate ratio [IRR], 0.97 [95% confidence interval {CI}, 0.81-1.16]). However, there was a large increase in the incidence of liver disease-related deaths in HCV-seropositive patients in adjusted models (IRR, 11.71 [95% CI, 6.42-21.34]). Among 2260 patients of known HCV serostatus initiating HAART, after adjustment, there was no significant difference between HCV-seropositive and -seronegative patients with respect to virologic response (relative hazard [RH], 1.13 [95% CI, 0.84-1.51]) and immunologic response, whether measured as a > or = 50% increase (RH, 0.94 [95% CI, 0.77-1.16]) or a > or = 50 cells/microL increase (RH, 0.92 [95% CI, 0.77-1.11]) in CD4 cell count after HAART initiation. CONCLUSIONS: HCV serostatus did not affect the risk of HIV-1 disease progression, but the risk of liver disease-related deaths was markedly increased in HCV-seropositive patients. The overall virologic and immunologic responses to HAART were not affected by HCV serostatus.