We report the successful treatment of a patient with plaque-stage mycosis fungoides with long-term and intravenous administration of recombinant human interferon-gamma (IFN-gamma) and discuss the possible mechanisms of this therapy. A 55-year-old female patient had been resistant to existing treatments and had suffered repeated exacerbations over a 5-year period. Four weeks after initiation of 2 x 10(6) U/day of IFN-gamma, a > 10% decrease in the affected surface area was noted. Twenty-two weeks after the administration of 228 x 10(6) U of IFN-gamma, complete remission (CR) was obtained. The CR continued for 13 weeks, but this was followed by an exacerbation. The second CR was obtained after the IFN-gamma dosage was increased to 16 x 10(6) U/week. The dosage was then gradually reduced by 2-4 x 10(6) U every 2 or 3 months. She was treated with a total dose of 2814 x 10(6) of IFN-gamma. She has been followed up for more than 6 years, and there has been no recurrence of mycotic skin lesions nor any visceral involvement. During therapy, no serious side-effects were noted. Long-term administration of IFN-gamma is useful for the treatment of patients with intractable mycosis fungoides. A gradual decrease in the dose of IFN-gamma is important for maintaining remission.