Thoracic aortic aneurysm, while relatively rare, is a potentially lethal disorder with a poor long-term prognosis if not treated. As the aortic diameter increases, wall tension becomes higher, leading to further expansion of the aneurysm. Consequently, this disease process is a "progressive and self propagating" phenomena, and the natural history of the disease is progressive expansion of the aneurysm with eventual rupture. Patients with descending thoracic aortic aneurysms frequently have concomitant major medical conditions, making them poor candidates for major surgical procedures. Concomitant conditions may include: hypertension, chronic obstructive pulmonary disease, congestive heart failure, and peripheral vascular disease, including abdominal aortic aneurysm, previous stroke and peripheral arterial occlusive disease.