The equilibrative nucleoside transporter 1 (ENT1) is the main protein involved in ribavirin cellular uptake. Polymorphisms at the ENT1 gene may influence ribavirin activity as part of hepatitis C virus (HCV) therapy. A retrospective study was conducted in 109 human immunodeficiency virus (HIV)-infected patients who were infected with HCV genotypes 1 or 4 who had received pegylated interferon (pegIFN)-ribavirin. Single nucleotide polymorphisms (SNPs) at the ENT1 gene were examined using TaqMan 5'-nuclease assays. In the study population, allelic frequencies at rs760370 were as follows: A3 (43 [39%] of 109 patients), AG (50 [46%] of 109 patients), and GG (16 [15%] of 109 patients). Achievement of rapid virological response was more frequent in GG carriers than in AA/AG carriers (50% vs 17%, respectively; P = .007). In multivariate analysis, the GG genotype (odds ratio [OR], 15.9; 95% confidence interval [CI], 2.8-92.2; P < .002), a baseline serum HCV-RNA level <600,000 IU/mL (OR, 45.7; 95% CI, 8.7-240.5; P <.001) and a serum ribavirin trough concentration >2.5 μg/mL (OR, 4.8; 95% CI, 1.3-17.1; P < .016) were associated with rapid virological response. When 2 or more of these factors were present, positive and negative predictive values of rapid virological response were 65% and 91%, respectively. In summary, a SNP rs760370A→G at the ENT1 gene influences the chance of rapid virological response to pegIFN-ribavirin therapy in HIV-infected patients with chronic HCV infection due to HCV genotypes 1 or 4, most likely modulating intracellular ribavirin exposure within hepatocytes.