Introduction The clinical applicability of early viral kinetics at week 4 in predicting sustained virological response (SVR) of pegylated interferon (peg-IFN) plus ribavirin (RBV) in HIV/HCV-coinfected patients is unclear. Our objective was to determine if rapid virological response (RVR) at week 4 of therapy with peg-IFN and RBV could predict SVR among HIV/HCV-coinfected patients. Methods HIV/HCV-coinfected patients in whom an HCV viral load determination had been carried out at week 4 of therapy were included in the study. The positive predictive value (PPV) and the negative predictive value (NPV) of RVR (undetectable serum HCV RNA at 4 week) for SVR were calculated in the study population. Receiver operating characteristic curves were calculated to determine the best cutoff of HCV RNA decrease to predict treatment failure. Results A total of 101 HIV/HCV-coinfected patients were included. RVR and SVR were observed in 39 (39%) and in 49 (48%) individuals, respectively. Of patients with RVR, 37/39 patients achieved SVR (PPV: 95%), whereas 50/62 individuals without RVR did not show SVR (NPV: 81%). The highest NPV (96%) was reached by using a cutoff level of HCV RNA decrease of 0.6 log₁₀. By applying this cutoff level, treatment could have been discontinued in 25 (25%) patients. Conclusions An undetectable serum HCV RNA determination at week 4 of treatment with peg-IFN plus RBV is a reliable predictor of SVR in HIV/HCV-coinfected patients. In addition, a decrease of HCV RNA less than 0.6 log₁₀ at this point of treatment could identify an appreciable proportion of individuals who will fail to achieve SVR.