Abstract: A 43-yr-old Japanese woman presented with mild anemia, leukocytosis and splenomegaly in May 1984. Splenomegaly and anemia gradually progressed. Sixteen years later, in October 2000, she developed inguinal lymphadenopathy. Biopsy of the lymph node revealed infiltration of blasts, megakaryocytes, fibroblasts and myeloid cells. Large blasts with basophilic cytoplasm with cytoplasmic projections appeared in the peripheral blood. These blasts were negative in peroxidase stain, positive in acid phosphatase and weakly positive in periodic acid-Schiff stain. Immunohistochemical staining with monoclonal antibodies revealed that these blasts were positive with anti-CD41 (glycoprotein IIb/IIIa) and negative with other monoclonal antibodies. So diagnosis of granulocytic sarcoma in megakaryoblastic transformation from chronic idiopathic myelofibrosis was made. A cytogenetic study revealed that bone marrow cells were 46,XX del(13)(q?) initially and additional abnormalities including der(5,5,11)(q11;q13)ins(5;?)(q11;?) were found when she developed megakaryoblastic transformation. Granulocytic sarcoma of megakaryoblastic transformation from chronic idiopathic myelofibrosis is a rare event. Immunophenotyping with monoclonal antibody for CD41(glycoprotein IIb/IIIa) confirmed the diagnosis.