A 17-year-old woman was suddenly seized with anterior chest pain and admitted to our hospital. Chest X-ray, computed tomography (CT) scan and magnetic resonance imaging (MRI) showed a mass shadow in the left middle lung field associated with the left pleural effusion and high serum carbohydrate antigen 19-9 (CA 19-9) levels (58.5 U/ml). We performed a total resection of the tumor with adherent parts of the left lung. The tumor was 6.5 x 3. 8 x 2.9 cm in size, and was made up of soft tissues, fluid and cystic components. The histological diagnosis was a mature cystic teratoma with perforation into the lung, which contained pancreatic tissues, hepatic cells, bronchial epithelium, serous glands and so on. The levels of fluid amylase and CA 19-9 were high. We report mediastinal mature teratoma perforating into the lung and discuss the mechanism of perforation.