Mediastinal tumor are frequently encountered in clinical practice. Hanten, in 1955, reported-2 adult patients with spontaneous rupture of mediastinal dermoid cysts into the pleural cavity and also, Thompson, in 1963, reported 2 child patients with spontaneous rupture of mediastinal teratoma into the pleural cavity. Mediastinal teratomas have also been reported rupture into other contiguous structures, such as the bronchus, aorta, pericardium, SVC and esophagus. This report presents an instance of spontaneous rupture of an anterior mediastinal teratoma into the right pleural cavity of a 43 year old female. Despite variable diagnostic procedures, the true nature of the lesion was not determined until a thoracotomy and window formation was performed for adequate drainage of empyema thoracis. Removal of the teratoma and mediastinal window formation resulted in complete cure.[KTCS 1977;1:59-64]