BACKGROUND: Low baseline serum hepatitis C virus (HCV)-RNA and clearance of viraemia at week 4 with pegylated interferon (pegIFN) plus ribavirin therapy predict sustained virological response (SVR) and enable a shorter duration of therapy in patients with chronic hepatitis C. It is unclear whether this applies to HIV/HCV-co-infected patients. PATIENTS AND METHODS: In the Peginterferon Ribavirin ESpaña COinfection (PRESCO) trial, 389 co-infected patients received pegIFN-alpha2a 180 microg/week plus ribavirin 1000-1200 mg/day. Patients with HCV-2/3 were treated for 6 or 12 months, whereas patients with HCV-1/4 were treated for 12 or 18 months. For each genotype, baseline HCV-RNA and rapid virological response (RVR), defined as under 50 IU/ml HCV-RNA at week 4, were evaluated as predictors of SVR in an 'on-treatment' analysis. RESULTS: Overall, SVR was achieved by 193 patients (49.6%), 68/191 (35.6%) with genotype 1, 110/152 (72.4%) with genotypes 2/3 and 15/46 (32.6%) with genotype 4. RVR was the best predictor of SVR regardless of HCV genotype. Only for HCV-1 patients, baseline HCV-RNA less than 500 000 IU/ml was also associated with SVR. In HCV-3 patients RVR had a positive predictive value (PPV) for SVR of 90%, with treatment for 24 or 48 weeks. The PPV of SVR for patients with RVR was 69% for HCV-1 and 83% for HCV-4. CONCLUSION: Undetectable HCV-RNA at week 4 is the best predictor of curing chronic hepatitis C in HCV/HIV-co-infected patients. In HCV-1 patients, baseline HCV-RNA also predicts response. HIV patients with HCV-3 and RVR may permit shortening therapy duration to only 24 weeks of pegIFN plus 1000-1200 mg ribavirin.