This case involves a patient with aortic leiomyosarcoma. A patient clinically suspected of type IIIa dissecting aortic aneurysm underwent surgery. The descending thoracic aorta was found to be filled with a soft, yellow tumor and was replaced with a woven Dacron graft. Microscopy of the surgical specimen revealed large, atypical spindle cells with numerous mitoses in bundles intersecting at 90 degrees, suggesting leiomyosarcoma. This diagnosis was confirmed immunohistochemical study. The postoperative course was uneventful and the patient was discharged on postoperative day 40; however, the patient developed bronchopneumonia due to malignant pleuritis 3 months postoperatively and died on postoperative day 103.