Six patients with documented systemic mast cell disease were enrolled in a 1-year, phase I study to determine the possible benefits of interferon alpha-2b (IFN-alpha). IFN-alpha therapy was begun at a dosage of 0.5 million units/day (MU/day) by subcutaneous injection and increased, as tolerated, to 3.0 MU/day. Subsequent dose modifications were made based on clinical tolerance and response. No immediate, adverse reactions to IFN-alpha occurred. Several patients showed symptomatic improvement. In two patients ascites resolved and did not recur. Two other patients reported improved energy levels and had decreased size of retroperitoneal, measenteric and retrocrural nodes. One patient failed to benefit and died shortly after completing 12 months of therapy. Bone marrow mastocytosis decreased by 5% to 10% after 12 months of therapy with IFN-alpha. Although five of the six patients had a decrease in the urinary excretion of 1-methyl-4-imidazole acetic acid, serum tryptase values did not appreciably change in any patient. Side-effects from IFN-alpha included hypothyroidism, thrombocytopenia and depression. It is concluded that although treatment with IFN-alpha was associated with a decline in bone marrow mastocytosis and reduced excretion of histamine metabolites, prolonged therapy may be needed and dose-limiting side-effects are frequent.