BACKGROUND: Chronic hepatitis C therapy in HIV patients is often penalized by more frequent premature treatment discontinuations. It is unclear what the relative contribution of more adverse events and/or early virological failures are. METHODS: PRESCO was a prospective, multicentre, comparative trial, in which 389 HIV/HCV-coinfected patients with CD4+ T-cell counts >300 cells/ml and elevated aminotransferases received pegylated interferon-alpha2a (peg IFN-alpha2a) 180 mg per week plus ribavirin (RBV) 1,000-1,200 mg daily. Patients with HCV genotypes 1 or 4 were treated for 48 or 72 weeks while HCV genotypes 2 or 3 carriers were treated for 24 or 48 weeks. Use of didanosine was not allowed. RESULTS: Sustained virological response (SVR) was achieved by 193 (49.6%), and was significantly greater in HCV-2/3 than in HCV-1/4 patients (72.4% versus 35%; P<0.0001). Premature treatment discontinuations occurred in 174 patients (44.7%). This was due to early virological failure in 66 (17%), serious adverse events in 32 (8.2%), loss-to-follow-up in 12 (3.1%) and voluntary withdrawal in 64 (16.4%). Only 10 patients (2.6%) stopped HCV therapy due to severe anaemia. Two patients stopped HCV medication due to symptomatic mitochondrial toxicity. There were no episodes of hepatic decompensation. CONCLUSIONS: Treatment with RBV 1,000-1,200 mg/day plus peg IFN-alpha2a is relatively safe and provided SVR in nearly half of the HIV/HCV-coinfected patients, twice as many amongst the HCV-2/3 than HCV-1/4 carriers. Avoidance of didanosine, limited use of zidovudine and therapy restricted to patients with CD4+ T-cell counts >300 cells/ml most probably explains the lower and different spectrum of serious adverse events in PRESCO compared with prior trials conducted in coinfected patients.