The myelodysplastic syndrome (MDS) is characterized by a high probability of leukemic transformation and frequently lethal infections or bleeding episodes. Up to now, no generally accepted form of therapy has been established for MDS. Previous trials with alpha-interferon (IFN-alpha) have shown some beneficial effects. We studied the effects of long-term application of IFN-alpha at higher dosages in patients with "low-risk" MDS. Ten patients were included in the study; eight were treated for a period of 6-36 months. IFN-alpha was administered at a median dosage of 9, 6, 4 mU/week during the first, second, and third year, respectively. Response was determined by the status of peripheral blood and bone marrow. Prolonged exposure to IFN resulted in a response rate of 3/10 (30%). In an additional case, disease progression was retarded during the third year of therapy. The incidences of infections and bleeding events subsided notably. After the withdrawal of IFN, hematological and clinical parameters rapidly deteriorated in some patients. The observed improvement of the patients' susceptibility to infections possibly prolongs their survival and seems to justify further trials on IFN treatment in patients with MDS.