Idiopathic myelofibrosis is a chronic myeloproliferative syndrome for which there is no effective treatment. The good results obtained with interferon in other chronic myeloproliferative syndromes have led their being tested in idiopathic myelofibrosis, but to date the experience is scarce. Four patients out of a total of 12 diagnosed with idiopathic myelofibrosis over a period of 3 years were selected for interferon treatment. Patients with low leukocyte or platelet counts or with contraindication for administration of the drug were excluded. Alpha-2b interferon was administered at an initial dose of 3 MU/day which was increased at 4-6 weeks to 5 MU/day in cases of insufficient response and if tolerance so permitted. In patients in whom favorable response was observed a maintenance schedule was initiated with low doses of interferon. Treatment was discontinued in two patients due to bad tolerance at 6 and 8 weeks of initiation of treatment with no response having been observed until that time. In the other patients favorable response was reported to interferon after 5 months of treatment with disappearance of the symptomatology, normalization of the hemo-peripheral values and a marked reduction of splenomegaly. This responses was accompanied by a decrease in bone marrow fibrosis in one case and total disappearance of the same in the other patient. Alpha-2b interferon constitutes an effective therapy for a selected number of patients with idiopathic myelofibrosis. Greater experience would allow the identification of the subgroup of patients who may benefit from this type of treatment.