Primary mediastinal choriocarcinoma is characteristically seen in young males presenting with the symptomes of cough, chest pale, and gynccomastia. A 33-year-old woman was admitted to the hospital because of severe dyspnea and chest pain which was aggravated rapidly 2 or 3 days ago. Posterior mcdiastinal mass measuring about 1 cm in diameter was seen in Chest P-A, left lateral view of chest, and chest CT. Serum $eta$-HCG level was markedly elevated up to 200, 000 mIxt. Whole body CT and other studies could not find any lesion on ovary and uterus. But, a single nodule nEeasuring about 1 cm in diameter was identified in the brain CT. The tumor cells (syncytiotrophoblastic cells) from resected mass revealed positivity on i histochemical staining for $eta$-HCG. She was treated with EMA-CO after resection of tumor, But, 7 months later, she was readmitted and showed cerebral hemorrhage due to metastatic choriocarcinoma. She was operated again for the brain tumor, and was doing well for further 7 months