To compare two interferon (IFN) schedules for the treatment of chronic hepatitis C, we followed 211 patients who received 3 million units IFN-alpha2b thrice weekly for either 6 months (group 1; 85 patients) or 12 months (group 2; 126 patients), with a median follow-up of 3.4 (0.1-8.4) and 4.2 (0.7-8.7) years, respectively. The biochemical and virological responses at the end of treatment were 34.1% and 16.5% versus 62.7% and 41.2% for the 6- and the 12-month regimens, respectively. Late biochemical responses (after the third month of treatment) occurred in 30.6% of responding patients, and they were not particularly associated with an adverse long-term treatment outcome. In a multivariate analysis, patients with a primary response were significantly more frequently infected with a non-1b HCV genotype (relative risk [RR]: 14.4), had been treated for 12 months (RR: 6.0), and had an early stage of liver fibrosis (RR: 5.2). Baseline serum HCV-RNA and ferritin levels also bore a significant, though weaker, association with a primary response. Using a set of pretreatment variables in a model of discriminant analysis, we could correctly predict the long-term virological outcome in 86.6% of the individual cases. At the end of follow-up, a biochemical and virological sustained response was observed in 14.1% and 11.8% versus 40.5% and 31% of groups 1 and 2, respectively. Significant predictors of a virological sustained response were a virological primary response (RR: 41.2) and the pretreatment level of serum HCV-RNA (RR: 10.3 per each 10(6)-Eq/mL decrease). Patients with a "good treatment profile," including an early stage of liver fibrosis, a non-1b genotype and serum HCV-RNA <0.35 x 10(6) Eq/mL, had a 66.7% rate of observed virological SR, compared with a zero response for those with the opposite, a "bad treatment profile." We conclude that a 12-month IFN treatment, along with a non-1b genotype and the absence of advanced stage of fibrosis, are the main determinants for the induction of a virological primary response in chronic hepatitis C. Such response, along with a low pretreatment serum HCV-RNA level, are the main predictors for a 4-year virological response to IFN.