A 29-year-old man was referred to our hospital complaining of chest pain and dyspnea on exercise. Computed tomography (CT) scan demonstrated a 9 × 8 cm mass in the left-sided anterior mediastinum. An ultrasonography showed no mass in the bilateral testis. A CT-guided biopsy was carried out twice but a few clusters of malignant tumor cells from uncertain histology could be obtained. On the basis of predominant midline tumor distribution, relatively young patient's age, male sex and increased serum hCG, β-hCG, AFP and LDH, we diagnosed primary mediastinal germ cell tumor (PMGCT), cT1N0M0S2, stage IIIB, with IGCCCG poor risk. He received standard cisplatin, etoposide and bleomycin (BEP) chemotherapy and the tumor markers returned to the normal level after two cycles of BEP. After he received four cycles of BEP, the tumor markers remain within normal range but a CT scan showed increased size of the mass. He underwent surgical resection of the mass. The pathologic examination revealed a remaining non-seminomatous tumor mainly composed of teratoma. He was planned to receive additional two cycles of BEP. Approximately one month after surgery, the serum LDH level rose. The hCG and AFP remained within normal ranges and a CT scan showed no evidence of recurrence of PMGCT. After he received an additional cycle of BEP, the serum LDH continued to rise for a month and finally large atypical cells appeared in the peripheral blood, which were count for 3% of the whole blood cells. Bilateral bone marrow aspiration resulted in dry tap, and bone marrow biopsy confirmed a diagnosis of acute megakaryocytic leukemia (M7) by the French–American–British (FAB) classification. He received induction chemotherapy of idarubicin–cytarabine for the leukemia.