The clinical, roentgenologic and laboratory findings in 124 patients with dissecting aneurysm of the aorta are reported. In 53 patients the dissection occurred in the ascending aorta ("proximal" dissection), and in 71 patients the site of origin was the descending thoracic aorta ("distal" dissection). Certain distinct clinical differences between the groups were apparent. Although hypertension was an important predisposing factor, it was significantly more common in distal dissection, as was atherosclerosis. Back pain and hypertension on hospital presentation characterized patients with distal dissection. Conversely patients with proximal dissection were younger and had a significantly higher incidence of Marfan's syndrome, cystic medial necrosis, anterior chest pain, pulse deficits, neurologic compromise, aortic insufficiency and congestive heart failure. In both groups, syncope appeared to correlate well with the occurrence of cardiac tamponade. Chest roentgenograms almost always showed an abnormal aortic contour. Aortic angiography, when performed, was usually confirmatory of the diagnosis.