The histological outcome in liver biopsies following 9 months of interferon alpha-2b treatment was assessed in detail in 19 patients with chronic posttransfusion non-A, non-B hepatitis (PTH-NANB) and compared with 12 untreated PTH-NANB patients. Fourteen (74%) treated and 7 (58%) control patients were reactive for antibodies against hepatitis C virus (anti-HCV). Liver biopsies taken before and after the 9-month period were scored numerically for portal inflammation, piecemeal necrosis (PMN) and fibrosis, without knowledge of whether the specimens came from control or treated patients. There were no score differences in the initial biopsies between the treated and control group. In the follow-up biopsies the treated group showed significantly less portal inflammation, PMN and fibrosis than the control group (p less than 0.05-0.01). When paired samples from the treated group were compared, significantly regressed portal inflammation, PMN and fibrosis were noted in the follow-up biopsies (p less than 0.05-0.001). The presence or not of anti-HCV antibodies in serum had no impact on the histological response to interferon treatment. We conclude that a 9-month course of interferon alpha-2b treatment significantly diminishes not only inflammation but also fibrosis in the liver of patients with PTH-NANB whether they are anti-HCV reactive or not.