A 13-year-old boy presented with an epidural thoracic granulocytic sarcoma manifesting as rapidly progressive paraplegia preceding clinical manifestation of acute myeloid leukemia (AML). Magnetic resonance imaging revealed a thoracic epidural tumor. He underwent emergent laminectomy and the tumor was totally resected. The initial histological diagnosis was malignant lymphoma. The correct diagnosis of epidural granulocytic sarcoma and AML was established based on cell-surface markers and a chromosomal study of the bone marrow cells. A combination of chemotherapy and bone marrow transfusion achieved complete remission of leukemia. No evidence of AML has emerged over the 18-month follow-up period. Granulocytic sarcoma should be considered in the differential diagnosis of an epidural mass in pediatric patients with or without acute leukemia. Immediate diagnosis and appropriate treatment are recommended to prevent leukemic transformation.