We assessed the efficacy of interferon (IFN) plus ribavirin over 24 or 48 weeks for the retreatment of patients with chronic hepatitis C who had relapsed or did not respond to a previous course of IFN. One-hundred and twenty patients (69 non-responders and 51 relapsers) were randomly assigned to receive IFN-alpha2b (3 million units thrice weekly) plus ribavirin (1,000-1,200 mg per day) for 24 weeks (group A: 58 patients) or 48 weeks (group B: 62 patients). Treatment was discontinued at week 12 if the alanine aminotransferase (ALT) level remained elevated. The rate of sustained response was 15.5% in group A and 37.1% in group B (P = 0.013). Relapsers treated for 48 weeks had a sustained response rate of 66.6% compared with a sustained response rate of only 25% in those treated for 24 weeks (P = 0.004). Moreover, a sustained response was seen in 14.3% of non-responders treated for 48 weeks and in 8.8% of those treated for 24 weeks (P = 0.71). Fifty-three per cent of patients with a normal ALT level and undetectable hepatitis C virus (HCV) RNA at week 12 had a sustained response compared with 14% of those who were HCV RNA positive at week 12 (P < 0.001). Independent predictive factors of sustained response were: therapy for 48 weeks (P = 0.0026), relapse after IFN treatment (P = 0.0006), loss of HCV RNA at week 12 (P = 0.0008) and HCV genotype non-1 (P = 0.024). Hence, in patients with chronic hepatitis C who failed to respond to a previous course of IFN monotherapy, combination therapy with IFN plus ribavirin for 48 weeks seems to be more effective than IFN plus ribavirin for 24 weeks.