Hepatitis C virus (HCV)-RNA clearance seems to occur more slowly in HIV/HCV-coinfected patients than in HCV-monoinfected subjects treated with pegylated interferon alpha (peg-IFN) plus ribavirin (RBV). As a consequence, concern has arisen over the feasibility of following the treatment rules applied to HIV-negative patients with chronic hepatitis C. A total of 89 HIV/HCV-coinfected patients who had fully completed a course of peg-IFN plus RBV were analysed. Of these, 29 (32.6%) reached sustained virological response (SVR). Reductions >2 logs in plasma HCV-RNA occurred in 52 (58%) patients at week 12 of treatment (early virological response; EVR). None of patients who showed HCV-RNA drops <2 logs at week 12 reached SVR (negative predictive value: 100%). The positive predictive value of EVR was 56%. On the other hand, relapses occurred in 19 (39.6%) out of the 48 patients who had negative HCV-RNA at the end of treatment, and there were no differences noted when comparing patients with HCV genotypes 2/3 and 1/4. In summary, the quantitative assessment of plasma HCV-RNA at week 12 predicts the chance of SVR using peg-IFN plus RBV in HIV-positive patients with chronic hepatitis C, as it does in HIV-negative individuals. Thus, discontinuation of anti-HCV therapy, which is associated with frequent side effects, might be warranted in HIV/HCV-coinfected patients showing HCV-RNA reductions <2 logs at week 12 of treatment. On the other hand, relapses in virological responders were unexpectedly high in HIV/HCV-coinfected patients when treatment was provided following the rules applied to HIV-negative subjects. This is particularly relevant for HCV genotypes 2/3, which only rarely relapse in HIV-negative patients. Therefore, extending therapy (for 12 months in HCV genotypes 2/3 and perhaps for 18 months in HCV genotypes 1/4) might be warranted in HIV/HCV-coinfected patients showing EVR.