A 35-year-old female was admitted with a large abnormal shadow in the left lung field on a chest X-ray. She was examined by chest tomogram, chest CT, MRI and bronchofiberscope. The CT and MRI showed a large tumor mass pressing the left lung and calcified deposits in the central part of the tumor. Bronchofiberscopic findings showed a stenosis of the left main bronchus. The clinical diagnosis was mature teratoma arising from anterior medistinum. She underwent a median sternotomy and the tumor was removed. The size of the resected specimen, filled with sebaceous material, hair and teeth, etc., was 14 x 12 x 10 cm (1,045 g). Histopathological examination of the specimen revealed a mediastinal mature teratoma. She has been well for 16 months postoperatively.