A 20-year-old male developed both coccygeal and leg pain and followed by rectocystic disturbance. Disc herniation between L5 and S was suspected and laminectomy was performed. At surgery, an easily curretable tumor occupied the epidural space from L5 to the end of the sacrum. In part, the tumor spread out of the vertebral canal and invaded the surrounding muscle tissue. This muscle tissue and part of the lamina were checked histologically. Initial blood analysis revealed 5% blast-like cells, but failed to confirm them as leukemic cells. Histologically, the tumor cells had round or oval nuclei with large nucleoli and scanty cytoplasm without granulocytic differentiation. Malignant lymphoma or Ewing's sarcoma was initially suspected, but the definite diagnosis was uncertain. Immunohistochemical staining with the PAP method and enzyme histochemistry revealed that the tumor cells were positive for lysozyme and naphthol ASD chloracetate esterase. Thus, granulocytic sarcoma was finally diagnosed. Electron microscopic findings supported this diagnosis. Subsequent karyotyping of bone marrow cells revealed 8; 21 translocation, thus the final diagnosis of this patient was myelodysplastic syndrome, refractory anemia with excess blast cells in transformation or acute myelogenous leukemia, M2, by the FAB classification.