A 32-year-old Japanese woman was referred to our hospital complaining of non-productive cough and dyspnea when in a dorsal position. Physical examination revealed left cervical lymph node swelling. Laboratory data showed HCG 17,700mIU/ml, betaHCG 130ng/ml, CYFRA 104ng/ml, AFP<2.0ng/ml, and LDH 802IU/l. A chest radiograph showed a giant mass shadow in the left hilum and many round mass shadows in both lung fields. Chest CT showed a mass shadow in the anterior mediastinum and many round mass shadows in both lung fields. There was no abnormality on abdominal and pelvic CT. A CT-guided transcutaneous lung biopsy specimen obtained from a mass in the left lower lobe revealed necrotized portions and atypical cells which were positive for HCG. We diagnosed primary choriocarcinoma of the mediastinum. The patient received 4 courses of the standard BEP regimen, resulting in partial response. This is the first reported case of primary choriocarcinoma of the mediastinum in a Japanese woman.