Primary granulocytic sarcoma (GS) is a rare disease defined by the absence of antecedent or concomitant leukemic cells in the bone marrow and the peripheral blood. Immunohistochemical staining for myeloperoxidase is necessary for a definite diagnosis. Otherwise, primary GS is often misdiagnosed as a malignant lymphoma or other malignancies. Primary GS is well known to frequently develop into acute myeloid leukemia (AML). Here we describe a 28-year-old woman with primary GS manifesting as an epidural tumor in the sacral region accompanied by meningeal dissemination. Fluorescence in situ hybridization analysis detected the AML1/MTG8 fusion gene in neoplastic cells obtained from her cerebrospinal fluid specimen and the epidural mass. The AML1/MTG8 fusion gene transcript was also detected by a nested reverse transcriptase-polymerase chain reaction analysis of mononuclear cells from the bone marrow, although leukemic cells were not recognized in a microscopical examination of the patient's bone marrow. Systemic chemotherapy with high-dose cytarabine followed by local radiotherapy was performed, and the patient clinically achieved a complete response. These molecular analyses provide a precise method of diagnosis, especially with respect to the French-American-British AML classification, according to the characteristic karyotypic alterations, and a patient consequently can quickly be given appropriate systemic chemotherapy as induction therapy.