BACKGROUND: Coinfection with hepatitis C virus (HCV) in human immunodeficiency virus (HIV) type 1-infected patients may decrease the effectiveness of highly active antiretroviral therapy. We determined the impact of HCV infection on response to highly active antiretroviral therapy and outcome among Danish patients with HIV-1 infection. METHODS: This prospective cohort study included all adult Danish HIV-1-infected patients who started highly active antiretroviral therapy from 1 January 1995 to 1 January 2004. Patients were classified as HCV positive (positive HCV serological test and/or HCV PCR results [443 patients [16%]]), HCV negative (consistent negative HCV serological test results [2183 patients [80%]]) and HCV-U (never tested for HCV [108 patients [4%]]). The study end points were viral load, CD4+ cell count, and mortality. RESULTS: Compared with the HCV-negative group, overall mortality was significantly higher in the HCV-positive group (mortality rate ratio, 2.4; 95% confidence interval [CI], 1.9-3.0), as was liver disease-related mortality (mortality rate ratio, 16; 95% CI, 7.2-33). Furthermore, patients in the HCV-positive group had a higher risk of dying with a prothrombin time <0.3, from acquired immunodeficiency syndrome-related disease, and if they had a history of alcohol abuse. Although we observed no difference in viral load between the HCV-positive and HCV-negative groups, the HCV-positive group had a marginally lower absolute CD4+ cell count. CONCLUSIONS: HIV-HCV-coinfected patients are compromised in their response to highly active antiretroviral therapy. Overall mortality, as well as mortality from liver-related and acquired immunodeficiency syndrome-related causes, is significantly increased in this patient group.