To quote Willis (1), “a teratoma is a true tumor or neoplasm composed of multiple tissues foreign to the part in which it arises.” Dermoid cyst is merely a synonym for benign cystic teratoma. After the ovary and testis, the commonest site for teratoma is the mediastinum, where the tumor is found almost invariably in the anterior portion and is almost always of the benign cystic type. All teratomas are probably congenital. Because of their inaccessible location and small size at birth, mediastinal teratomas are usually not discovered much before the twentieth year of life, and frequently not until after the age of thirty. Benign cystic teratomas grow slowly and progressively, in the course of years accumulating large quantities of sebaceous fluid, teeth, hair, skin, bone, cartilage, etc. In adult life, even when the tumor attains considerable size, symptoms are usually absent. A mediastinal dermoid may be an incidental finding in chest films taken for unrelated symptoms or during a routine survey. Symptoms may not develop until spontaneous rupture occurs. This latter event is probably the result of high intracystic pressure from accumulated fluid or of necrosis of the cyst wall from interference with its blood supply. One of these factors, or both, probably leads to rupture into a bronchus, into the pericardium, aorta, superior vena cava, pleural cavity, the neck, or onto the chest wall. Reports of spontaneous rupture of mediastinal teratomas are scarce. The writers could find but one recorded case of spontaneous rupture of dermoid cyst into the pleural cavity (Wheeler, 2). Recently, however, they have encountered 2 patients, each with a dermoid cyst of the anterior mediastinum which spontaneously ruptured into the right pleural cavity. Case Reports Case I: D.O., a 19-year-old unmarried white female, was first admitted to the hospital on Sept. 27, 1948, because of sudden onset of severe right chest pain and shortness of breath of three hours duration. Except for occasional “colds,” she had been in good health until the present illness. She was well developed and well nourished, but in acute disstress, dyspneic, and slightly cyanotic. Abnormal findings were confined to the inferior third of the right chest, where there were decreased breath sounds and percussion dullness. The temperature was 100.2° on admission and remained normal thereafter. The blood count was normal and the blood Kahn test negative. The initial chest roentgenogram, Oct. 2, 1948, demonstrated a moderate amount of fluid in the base of the right chest and a right anterior mediastinal soft-tissue mass was partially revealed (Fig. 1).