Acute myelofibrosis is a rare but distinct accelerated variant of agnogenic myeloid metaplasia that is characterized by marked anemia, peripheral blood myeloblastosis and normoblastosis, a lack of teardrop poikilocytosis, and prominent myelofibrosis. There is usually no palpable hepatosplenomegaly or lymph node enlargement. The clinical course is remarkable short. We describe a 63-year-old man who presented with idiopathic acquired sideroblastic anemia and subsequently developed acute myelofibrosis. Intensive polychemotherapy with vincristine, cytosine arabinoside, and prednisone and a later trial of oxymetholone therapy were ineffective. He died 134 days after the diagnosis of acute myelofibrosis was established. The 11 previously reported cases of acute myelofibrosis are reviewed, and the relationships of acute myelofibrosis to other myeloproliferative disorders and to idiopathic acquired sideroblastic anemia are discussed.