Summary. In spite of the availability of hepatitis B vaccine, acute hepatitis B continues to be a worldwide problem for which no specific therapy is available. We investigated the safety and the effectiveness of recombinant interferon-α2b (rIFN-α2b) in the treatment of acute hepatitis B by determining overall severity and duration of symptoms, time required to clear viral antigens and hepatitis B virus (HBV) DNA, and titre of antibodies to hepatitis B surface antigen (HBsAb), 24 weeks after the onset of therapy. One hundred patients were randomly assigned to treatment with either 3 million units (MU) (n= 34) or 10 MU (n= 33) rIFN-α2b or to placebo (n= 33), three times weekly for 3 weeks. Follow-up was for 24 weeks. A significantly shorter duration of the symptoms and signs of acute hepatitis was observed in patients who received 3 MU rIFN-α2b compared with those who received 10 MU rIFN-α2b or placebo. Twenty-one weeks post-therapy, patients treated with 10MU rIFN-α2b showed a significantly higher geometric mean HBsAb titre than those treated with placebo (85.1 vs 35.5 IU1-1, P < 0.05), rIFN-α2b administration was well tolerated even in jaundiced patients. No serious side-effects were observed necessitating reduction in dose or discontinuation of the drug. The effect of rIFN-α2b on transition of HBV infection to chronicity could not be evaluated in this trial because such an unfavourable course was not seen in any of the treated or the control patients. In conclusion, rIFN-α2b was safe in acute hepatitis B, and at low dose was found to ameliorate symptoms and to shorten significantly the duration of illness.