A 56-year-old woman was admitted with pyrexia, cough, and dyspnea on August 21, 1991. Physical examination revealed anemia in the palpebral conjunctivas and moist rales at the right lower lung field. Neither the Liver nor spleen was enlarged. Examination of the peripheral blood showed a hemoglobin level of 8.1 g/dl, a platelet count of 14.8 x 10(4)/microliters, and a white blood cell count of 2,800/microliters, with 7% blasts and 8% megakaryocytes. Tear drop-like erythrocytes, agranular neutrophils, and erythroblasts were also seen in the peripheral blood. Examination of the bone marrow showed 15% peroxidase positive blasts, and many micromegakaryocytes. Cytogenetic studies for bone marrow cells revealed the existence of the Philadelphia (Ph1) chromosome. Bone marrow biopsy showed normal cellularity with increase of megakaryocytes and advanced myelofibrosis. Breakpoint cluster region (bcr) rearrangement analysis using the peripheral blood mononuclear cells revealed M-bcr rearrangement. According to the Hannover classification for myeloproliferative disease, she was diagnosed as having CML with advanced myelofibrosis followed by CML with megakaryocytic increase. Since she had neutrocytopenia and severe infectious disease, she received a subcutaneous injection of 125 micrograms of G-CSF. Not only increase of the white blood cell count, but also disappearance of blasts, improvement of anemia, increase of the platelet count, and improvement of myelofibrosis were observed.