[en] BACKGROUND/AIMS: Treatment of chronic hepatitis C with interferon can be ineffective due to relapse. We aimed to reduce the 40% relapse rate of 6 months interferon-ribavirin combination therapy by prolonging treatment to 18 months. METHODS: Three hundred patients with treatment-naive hepatitis C, were randomized to 18 months combination therapy with interferon (3MU tiw) and ribavirin (1000-1200 mg/day), 18 months interferon combined with placebo, or 6 months combination therapy with interferon and ribavirin, in a double blinded manner. All 295 patients who received at least one dose of treatment were included in the intention to treat analysis. RESULTS: At the end of treatment, HCV RNA was undetectable in 55 and 49% of those on 6 and 18 months combination therapy, respectively, versus 26% of those on monotherapy (P<0.001). The relapse rate was 38% for 6 months combination therapy, 38% for 18 months monotherapy, and only 13% for 18 months combination treatment (P=0.002). The sustained response rates were 34% for 6 months combination therapy, 16% for 18 months monotherapy and 43% for 18 months combination therapy (P<0.05). CONCLUSIONS: Reduction of relapse rates to 15% or less is feasible by prolongation of interferon-ribavirin treatment to 18 months.