A 70-year-old man, who had been diagnosed as primary myelofibrosis in 1987 at Tokyo Women's Medical School, was admitted to our hospital because of left hip-joint pain in May 1988. Physical examinations revealed marked hepatosplenomegaly and multiple reddish papules on the skin. The peripheral blood showed marked leukoerythroblastosis and severe anemia with poikilocytosis. Bone marrow aspirations were dry tap. Needle biopsy of iliac crest showed a diffuse fibrosis. Biopsy of the papules showed an extramedullary haematopoiesis. He was treated with low dose Ara-C for seven days. Although hepatosplenomegaly and skin papules were reduced, he died of pneumonia a month later. At autopsy, leukemic cells massively infiltrated into the multiple organs, including bone marrow. The cells were identified with megakaryoblast, as those were positive for factor VIII related antigen. As far as we know, this is the third case of primary myelofibrosis transformed into acute megakaryoblastic leukemia.