BACKGROUND: Relapse after achieving virologic response to anti-hepatitis C virus (HCV) treatment considerably reduces sustained virologic response rates. It is unclear what the main predictors of relapse in HCV/HIV-coinfected patients are. PATIENTS AND METHODS: The Pegasys Ribavirina España Coinfección (PRESCO) study evaluated short and extended duration of treatment for chronic hepatitis C using pegylated interferon (peg-IFN)-alpha2a at a dose of 180 microg/wk plus weight-based ribavirin (RBV) at a dose of 1000 to 1200 mg/d in HIV-infected subjects. Patients with HCV-2/3 were treated for 6 or 12 months, and patients with HCV-1/4 were treated for 12 or 18 months. RESULTS: Of 389 patients included in the trial, end-of-treatment response was achieved by 262 (67.3%): 106 with HCV-1 (55%), 137 with HCV-2/3 (90%), and 19 with HCV-4 (41%). Six patients were lost to follow-up after completing therapy. Of the remaining 256 patients, 62 (24%) relapsed: 33% of HCV-1 patients, 18% of HCV-2/3 patients, and 21% of HCV-4 patients. In multivariate logistic regression analysis, baseline serum HCV RNA level > or =500,000 IU/mL (relative risk [RR] = 4.81, 95% confidence interval [CI]: 1.52 to 15.22; P = 0.008) and lack of rapid virologic response, defined as undetectable HCV RNA level at week 4 (RR = 2.94, 95% CI: 1.22 to 7.09; P = 0.02) were the best independent predictors of HCV relapse. Use of concomitant antiretroviral therapy also predicted relapse (P = 0.04), and a trend toward a higher relapse rate was recognized for HCV genotypes 1 and 4 versus genotypes 2 and 3 (P = 0.08). Extended treatment did not result in a lower incidence of relapse, at least for HCV genotypes 2 and 3. CONCLUSION: High baseline serum HCV RNA level and lack of undetectable viremia at week 4 are the most significant predictors of relapse in HCV/HIV-coinfected patients treated with peg-IFN plus weight-based RBV.