This case involves a man in his fourties. An anterior mediastinal tumor was detected by a previous doctor who he had visited because of cough, and he was referred to our hospital for examination. A tracheal stent was inserted, as the trachea had narrowed because of the tumor. Although the diagnosis had not yet been confirmed, his respiratory condition was unfavorable, and chemotherapy was initiated immediately. A tumor biopsy was performed, and the result was a small round cell tumor; this did not help to confirm the diagnosis. After chemotherapy, the tumor shrank; however, tumor re-growth was observed. Because atypical cells were observed in the peripheral blood, he visited the Division of Hematology in our Department of Internal Medicine. As a result of examination, he was diagnosed with myeloid/natural killer (NK) cell precursor acute leukemia. Remission induction therapy was administered, but sepsis occurred, and the patient died. Myeloid/NK cell precursor acute leukemia is a rare disease that involves the formation of extranodal lesions in the mediastinum. It is assumed that respiratory physicians are first to examine mediastinal tumors for the possibility of myeloid/NK cell precursor acute leukemia. Myeloid/NK cell precursor acute leukemia needs to be noted as a differential diagnosis of mediastinal tumors.