Interferon-beta (GKT-beta) was administered to a patient with adult T-cell leukemia (ATL), presumably of acute type, with a marked organ infiltration and increases in serum calcium, LDH and bilirubin. The therapy produced a fall in the number of peripheral blood abnormal lymphocytes, disappearance of jaundice, reduction in the degree of hepatomegaly and splenomegaly and a remarkable improvement of general condition with a performance status of grade 3 improving to grade 1. The response has been maintained for more than seven months. Dose, schedule and criteria for drug discontinuation are matters for further investigation.