Thoracic aortic aneurysms were detected in 72 residents (44 women and 28 men) in a stable midwestern community over a 30-year period, for an age- and sex-adjusted incidence of 5.9 new aneurysms per 100,000 person-years. The incidence was equal in both sexes and decreased slightly over the 30 years. Ages ranged from 47 to 93 years (median 65 years for men and 77 years for women). The ascending aorta was involved in 37 patients, the aortic arch in 8, and the descending aorta in 27. Pathologic examination was performed in 51 patients. The cause was aortic dissection in 27 patients (53%), atherosclerosis in 15 (29%), aortitis in 4 (8%), cystic medial necrosis in 3 (6%), and syphilis in 2 (4%). All autopsied patients had pathologic evidence of significant hypertension. Eleven patients (25%) had concomitant abdominal aortic aneurysms. Rupture occurred in 53 patients (74%) and 50 died. Thirty-seven of these patients had no prior diagnosis of aneurysm. The median interval between diagnosis and rupture in the 16 remaining patients was 2 years (range 1 month to 16 years). Ninety-five percent of aortic dissections ruptured and 51% of nondissecting aneurysms ruptured. The actuarial 5-year survival for all 72 patients was 13%; for patients with aortic dissection, 7% and for patients without dissection, 19.2%.