We report the case of a 14-year-old female with acute myeloid leukemia (AML) and myeloid sarcomas (MS) in the anterior mediastinum and around numerous bones. Laboratory tests showed a white blood cell count of 4.0 × 10(9)/l with 7.0 % blasts. Computed tomography revealed a mediastinal mass and pleural effusion; pleural effusion cytology was negative for malignant cells. In addition, disseminated intravascular coagulation (DIC) was present. Following DIC therapy, thoracoscopic and bone marrow biopsies were performed. Immunostaining and surface marker analysis revealed that the blast cells were positive for cytoplasmic myeloperoxidase, CD4, CD7, CD33, CD44, CD117, and HLA-DR, but negative for CD34 and CD56. Karyotype was normal. MS associated with AML was diagnosed. Multidrug chemotherapy for AML was completely ineffective, and MS continued to progress. Immunohistochemistry revealed that the blasts were negative for asparagine synthetase (AS); therefore, chemotherapy including L: -asparaginase was initiated. After the first administration of L: -asparaginase, the patient's condition improved; however, she subsequently developed tumor lysis syndrome and sepsis, which eventually led to death. Aggressive MS in childhood is rare and refractory to existing AML chemotherapy. Chemotherapy including L: -asparaginase may prove to be effective in such cases, especially those in which blast cells show negative AS expression.