A 45-year-old man was admitted with high fever and leukocytosis in August 1993. The diagnosis of acute myelogenous leukemia (AML; M2) was made on the basis of morphological, cytochemical and immunological characteristics of the blasts in the bone marrow. The induction therapy with BHAC, daunorubicin, 6-MP was unsuccessful in achieving remission; the bone marrow biopsy specimen revealed the proliferation of the remaining leukemic cells and massive fibrosis accompanied with unusual megakaryocyte-like giant bizarre cells. These megakaryocyte-like giant cells were positive for myeloperoxidase and CD34, but not GPIIIa and factor VIII, indicating that those were derived from myelogenous stem cells. Following the low-dose Ara-C therapy, improvement of fibrosis and disappearance of these giant cells were observed in the bone marrow. After the reinduction therapy with high-dose Ara-C and MIT against markedly increased blasts, the patient died of systemic fungal infection. The presence of myelofibrosis and giant atypical blasts might allow resistance to therapy and poor prognosis.