Several controlled trials have shown that administration of interferon-alpha for some months with a dosage about 3 million units three times weekly induces normalization of alanine aminotransferase (ALT) activity and seroconversion of patients in about 40% of cases with B-virus-induced hepatitis at the end of treatment. The aim of this prospective open multicentre study is to give further data about the dosage and the long-term administration of alpha-interferon. Eight liver units in universities and hospitals have been involved in this trial. Twenty-one patients with chronic B hepatitis were selected for study. Therapy protocol: The patients with chronic B hepatitis were treated with interferon-alpha(2b) (Intron A) for one year. If the patients had no response after three months, the dose was increased After one year of therapy, the patients were observed for another six months. Efficacy of the therapy: Sustained complete remission occurred in ten cases (47.6%); partial remission in five; four patients had no response; in one case, the disease had progressed; and one patient dropped out. Side-effects, apart from a flu-like syndrome, occurred in only a very few cases. Conclusion: Long-term therapy with interferon-alpha(2b) for one year produced sustained complete remission in about half of the patients. In non-responders after three months' therapy, increasing the dose of interferon had a beneficial effect. On the basis of these data, long-term therapy and higher dosage in non-responders can be recommended.