The authors report on two female patients aged 12 and 14 years, who spontaneously developed a rupture of benign mediastinal cystic teratoma into the right pleural cavity. They presented with acute onset of severe chest pain and respiratory distress. The tumors were completely resected by thoracotomy. The serum and pleural fluid levels of carcinoembryonic antigens, CA-125 and CA19-9 were invariably elevated, then decreased to normal range after the surgical resection. Rapid diagnosis of this extremely rare complication is important because it may progress to a life-threatening condition.