During the past 15 years there has been an excellent opportunity to study the abnormal x-ray shadows that appear in the mediastinum in patients seen at the Indiana University Medical Center. It is difficult to establish the true incidence of mediastinal tumors in a hospital population; however a total of 115 tumors were seen during a period when 123,753 patients were admitted to this Medical Center. The protocol observed for each patient included a careful history, physical examination, and x-rays of the chest. The x-ray studies included the usual P-A chest, lateral film, and cardiac fluoroscopy which included a barium swallow. Planograms were considered when mass lesions were indistinct. When the aorta or aortic arch could not be separated from the shadow, an aortogram or cardioangiogram was performed. A routine electrocardiogram proved valuable on several occasions. A careful search of entire organ systems was used to exclude the possibility of