Acute myelofibrosis (AM) or malignant myelosclerosis is a myeloproliferative syndrome in which bone marrow fibrosis is associated with a proliferation of immature myeloid cells. In 4 patients with typical AM, investigated by the platelet-peroxidase reaction at ultrastructural level, the blast cells were megakaryoblasts. One patient, treated with the drug combination DAT [daunorubicin, cytosine arabinoside, 6-thioguanine], achieved a complete remission of 5 mo. duration. This study supports the view that megakaryoblastic leukemia is the most frequent underlying cause of AM and proposes that it should be classified as a form of acute myeloid leukemia.