Hepatitis C virus (HCV)/human immunodeficirency virus (HIV) coinfection poses a difficult therapeutic problem. Response to HCV-specific therapy is variable but might be influenced by host genetic factors, including polymorphisms of cytokine genes. Here, we studied whether interleukin-6 (IL-6) C174G gene polymorphism affects the response to antiviral treatment in HCV-infected HIV-positive subjects. We determined IL-6 genotypes in HIV-positive patients with acute (n = 52) and chronic (n = 60) hepatitis C treated with pegylated interferon-α. Two hundred ten HCV monoinfected, 197 HIV monoinfected, and 100 healthy individuals were studied as controls. Patients were classified into high and low producers according to IL-6 genotypes. Rates of sustained virological responses (SVRs) were compared between the IL-6 genotypes. Signal transducer and activator of transcription three phosphorylation was analyzed by Western blot in HCV core-transfected human hepatoma cell line (HUH7) cells. Distribution of IL-6 genotypes did not differ significantly between the study groups. SVR was achieved in 63% of HIV/HCV coinfected patients. Carriers of the IL-6 high producer (HP) genotype had significantly higher SVR rates than patients with an IL-6 low producer genotype (70.1% versus 52%; P < 0.002). This effect was seen in both HIV-positive patients with acute (74% versus 33%; P < 0.05) and chronic (66% versus 33%; P < 0.05) hepatitis C. Multivariate analysis confirmed IL-6 HP carriage as an independent positive predictor for SVR (Odd's ratio 6.1; P = 0.004). This effect corresponds to the in vitro observation that in HCV core-transfected HUH7 cells, IL-6 overcomes the HCV core-mediated inhibition of STAT3 activation. Conclusion: Response rates to HCV-specific treatment are higher in HCV/HIV-positive patients carrying the IL-6 HP genotype, which might be because of IL-6 mediated STAT3 activation. Copyright © 2007 by the American Association for the Study of Liver Diseases.