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One hundred eight patients with spontaneously developing thoracic aortic dissection were seen between 1966–1973, 78 of whom had acute dissection and 30 chronic. The age (49 vs 60 yrs) and incidence of hypertension (32% vs 71%) were significantly lower in the 56 patients in whom dissection originated in ascending aorta than in the 52 patients in whom the dissection originated in the upper descending aorta. The mortality rate in medically treated patients with acute ascending aortic dissection was 88%. Cardiac tamponade was the major cause of death. The mortality rate was significantly lower in those who were treated surgically (24%). Fifteen (54%) of the patients with ascending aortic dissection and significant aortic incompetence did not have aortic valve replacement and only two subsequently (53 and 92 months later) required valve replacement. Although the initial mortality in patients with acute descending aortic dissection treated medically and surgically was similar, the long term survival rate was higher in the surgically treated group. We conclude that ascending aortic dissection and descending aortic dissection have different clinical profiles and prognoses. Immediate surgical intervention is indicated in patients with acute ascending aortic dissection. Patients with acute descending aortic dissection can be treated medically initially followed by early elective operation.