33 patients with biopsy-proven chronic non-A, non-B posttransfusion hepatitis (NANB PTH) were randomized 2:1 to treatment with interferon alpha-2b (Introna) or to controls. The treatment group received 3 MU interferon 3 times weekly subcutaneously for 36 weeks. 22/33 (67%) patients were reactive for antibodies against hepatitis C virus (anti-HCV). 11/19 (58%) treated patients versus none of the 12 controls had a complete response with normalization of serum alanine aminotransferase levels (p less than 0.001). Another 4/29 (21%) treated patients had a partial response which was also seen in 4/12 (33%) controls; 4 treated patients were nonresponders, all with chronic active hepatitis. Nonresponders had a significantly higher mean weight than responders (p less than 0.05) and tended to have a longer duration of their disease before therapy. During the 6-month follow-up period post treatment 4/11 (36%) complete responders had a sustained response and 7/11 (64%) relapsed. All who were retreated responded again. We conclude that a majority of patients with chronic NANB PTH will respond to 9 months interferon alpha-2b treatment, but that only 1 out of 3 will have a sustained response 6 months post treatment, and that the reactivity for anti-HCV was not correlated to the outcome of therapy.