A case of a primary undifferentiated sarcoma of the descending thoracic aorta in a 75-year-old man is reported. Intraluminal growth and occlusion produced congestive heart failure and renal failure. The thoracic aorta was relatively normal on chest radiography so that the lesion was undiscovered until the autopsy. To our knowledge, it is the first case of a primary undifferentiated sarcoma of the thoracic aorta, and the first undifferentiated aortic sarcoma to be examined with immunohistochemistry and electron microscopy. Electron microscopy and immunohistochemistry verified the sarcomatous nature of the undifferentiated neoplasm.