Hepatitis C virus (HCV) is responsible for the majority of cases of post transfusion non-A non-B (NANB) hepatitis in thalassaemia major (TM)1. Fifteen multi-transfused TM patients with serological, biochemical, histological and molecular biological evidence of HCV infection have been treated for six months with recombinant alpha interferon (IFN). Eleven (73%) responded, 8 (53%) had complete response (CR), 3 (20%) partial response (PR) and 4 (27%) did not respond (NR) to IFN. Natural killer (NK) cell activity 24 hours after the first dose of IFN was significantly increased in responders as compared to nonresponders. Liver histology showed an overall reduction of portal inflammation and periportal necrosis in the responding patients. HCV RNA disappeared from serum in 8 (15) responders and partial responders. Non responders remained positive. HCV RNA was tested and found to be positive in liver tissue material in 7 patients, five of those were re-tested after IFN treatment. Two became negative (both CR) 3 remained positive despite biochemical response to IFN. The degree of induction of peripheral blood mononuclear cell 2'5' oligoadenylate synthetase messenger RNA (2-5 OAS mRNA), an enzyme induced by IFN, after the first dose of IFN did not correlate with response neither was any significant interaction with cytokines observed; tumour necrosis factor (TNF), interleukin-1 (IL-1) and CD4:CD8 ratios did not change. We conclude that IFN should be given to all TM patients with chronic active hepatitis due to HCV.