The autopsy of a 28-year-old Japanese male patient, which revealed immature teratoma combined with choriocarcinoma in the mediastinum, is presented. The tumor, which was removed after chemotherapy, was localized in the superior-anterior mediastinum. Chemotherapy was performed several times after operation. However, HCG increased again, and a roentgenogram revealed metastatic shadows in both lungs. The metastatic tumors in the lungs showed only the choriocarcinoma.