Thirty patients with chronic non-A, non-B hepatitis (24 male, six female; median age 38 years, range: 15-68 years) were treated with recombinant interferon alfa-2b for 1 year. Treatment was started with 5 million units interferon alfa-2b daily for 2 weeks followed by 2 million units daily for another 2 weeks. Further doses were titrated according to alanine aminotransferase values. After 1 year, treatment was stopped and a follow-up biopsy was obtained. Thereafter, patients were followed for 6 months. Of the 24 patients who completed the 1-year treatment period, 14 (58%) had normal alanine aminotransferase values at the end of the study, eight of whom showed transient increases while on treatment. In another seven (29%), alanine aminotransferase levels decreased by more than 50% of pre-treatment values but remained above the normal range. Biopsies at the end of treatment showed a complete disappearance of inflammatory activity in four and a marked improvement in eleven other patients. The results of this study indicate that a 1-year treatment with recombinant interferon alfa-2b of patients with non-A, non-B hepatitis was very effective at normalizing or improving serum transaminases and liver histology. However, the overall relapse rate was 57%, with relapse occurring in a greater proportion of patients with temporary breakthroughs during therapy (requiring dosage increase), and particularly of patients with only a partial response to treatment (serum transaminases decreased by greater than or equal to 50%). Thus, further studies are needed to establish the optimal dose and duration of treatment to induce a complete resolution of the disease.