Adult T-cell leukemia (ATL) is one of the most difficult diseases to treat because of severe underlying immune deficiency and metabolic disturbance. Interferon has potent antiviral, antiproliferative, and immunomodulating properties, and therefore, this may be a good agent to treat such immune deficient patients with peripheral T-cell leukemia. During a period from April 1984 to August 1985, six patients were treated with interferon-beta (IFN-beta), and interferon-gamma (IFN-gamma) was given to five patients. Three patients achieved partial remission by IFN-beta administration with a response duration of 1, 1.5, and 12 months respectively, whereas one complete remission and two partial responses were experienced by IFN-gamma treatment with 4, 4, and 2 months of response. Side effects of IFN-beta were similar to those of IFN-gamma including fever, chills, fatigue, mild hematologic depression, and transient hepatic enzyme abnormalities. These promising results warrant further well-designed clinical trials including combination with other agents or modalities of treatment.