The purpose of this study is to evaluate the long-term results of medical treatment for a dissecting aorta and to detect the risk factors that determine the prognosis of medically treated patients. During the past 10 years, 228 patients with aortic dissections were admitted to our hospital and affiliated hospitals. One hundred thirty-four patients, including 60 with proximal type (Stanford, type A) and 74 with peripheral type (Stanford, type B) dissections, were treated by medical means alone. The survival rates of medically treated patients with type A dissections at 24 hours, 2 weeks, and 5 and 10 years after the onset of the disease were 72, 43, 34, and 28%, respectively, and the survival rates in type B dissections were 100, 92, 76, and 56%, respectively. The risk factors that determine poor prognosis in the acute phase of dissections were type A dissection and serious complications (rupture of the aorta, shock, cerebral accident, myocardial infarction, severe aortic regurgitation, renal failure, mesenteric infarction, and arterial occlusion in the extremities). The risk factors in the chronic phase were serious complications, excluding shock and rupture in the acute phase, the large diameter of the dissecting aorta, and increasing age. These results show that emergency surgical intervention is indicated in the patients with acute type A dissections and in those who had acute type B dissections with these serious complications. Medical treatment may be tentatively recommended for the patients with uncomplicated type B dissections until the operative death rate in these patients becomes less than presently identified.