A 22-year-old man was diagnosed with isolated mediastinal myeloid sarcoma which radiologically mimicked primary mediastinal lymphoma. Despite administration of standard remission induction chemotherapy with daunorubicin and cytarabine, and three cycles of intensive high-dose cytarabine-based consolidation, chemo-resistant hypermetabolic lesions were persistently detected in the highest mediastinum and in the supraclavicular area. However, complete remission and long-term survival were achieved by curative radiotherapy followed by unrelated allogeneic stem cell transplantation.