The biochemical and histological long-term outcome of interferon alpha-2b treatment for chronic posttransfusion non-A, non-B/C hepatitis was evaluated in a randomized study. 4/19 treated patients had a sustained response with normal serum alanine aminotransferases (s-ALAT) levels during follow-up, at present for 18-20 months after the end of interferon treatment. None of 11 responders with biochemical relapse normalized their s-ALAT levels during 1 year follow-up after treatment. Histological changes were assessed by a scoring system. The scores for portal inflammation, piecemeal necrosis and fibrosis were essentially unchanged in all treated patients between biopsies taken at the end of treatment and 1 year later. Six non-responders to 3 million units (MU) alpha-2b interferon thrice weekly (t.i.w.) were given 6 MU t.i.w. for at least 8 weeks. None normalized the s-ALAT levels during treatment with the higher dose, instead the side effects were much more pronounced, an obstacle to the usefulness of higher interferon doses. The biochemical non-responders had higher pretreatment histologic inflammation scores indicating a more severe infection. They also had a higher body weight and seemed to have prominent macrovesicular steatosis more often than responders, a finding that could contribute to raised aminotransferases, thereby in some of the non-responders, masking a positive biochemical effect of interferon treatment.