A 42-hour-old male newborn was referred to our hospital with the complaints of life-threatening respiratory distress. A large mass was observed on posterior-anterior chest radiography. A 10 x 10 cm mass was completely excised from the anterior mediastinum and right hemitorax via right thoracotomy. Histopathologic examination of the mass revealed as immature teratoma. Mediastinal immature teratoma may cause life threatening respiratory obstruction and must be promptly diagnosed and treated.