We compared sustained virological response (SVR) in chronic hepatitis C patients with severe fibrosis treated with pegylated interferon (Peg-IFN) alpha-2b 1.5 microg/kg/week or 0.75 microg/kg/week in combination with ribavirin 800 mg/day for 48 weeks. This was a multicentre randomized controlled study. SVR was observed in 44.5% (45/101) of patients treated with the standard dose of Peg-IFN and 37.2% (38/102) of patients treated with the low dose (NS). In patients with genotypes 1, 4 and 5, SVR was observed in 25.0% of patients who received the standard dose and 16.9% of patients who received the low dose of Peg-IFN (P = NS). In patients with genotypes 1, 4 and 5 and low viraemia, SVR was obtained in 27.3% of patients treated with the standard dose and 25.8% of patients treated with the low dose (P = NS). In the high-viraemia subgroup, SVR was obtained in 24.0% and 9.1% of patients, respectively. In patients with genotypes 2 and 3, SVR was similar in both groups (73.2%vs 73.0%). Thus, (1) patients with genotypes 2 and 3 and severe fibrosis can be treated with low dose of Peg-IFN and ribavirin, (2) this study suggests that patients with genotypes 1, 4 and 5 and high viraemia could receive a standard dose of Peg-IFN associated with ribavirin for 48 weeks, (3) side effects limit the efficacy of the treatment with standard dose of Peg-IFN in patients with genotypes 1, 4 and 5 and low viraemia, (4) more studies are needed for patients with genotype 2 or 3 to define the optimal duration (24 or 48 weeks) in patients with severe fibrosis.