Between 1945 and 1961 50 patients with aortic dissection were seen at the Mayo Clinic. Survival information for prognostic purposes was available in 40 patients and revealed that more than half of the patients were dead within a week. Of 15 patients who survived the acute phase (2 weeks) more than half were dead within a year. The cause of death was determined by necropsy in 24 patients and 22 died of external rupture of the dissecting aneurysm. In 11 patients a site of re-entry in the dissection was found and in all of these patients external aortic rupture had occurred also. The surgical approach to the treatment of aortic dissection is discussed in the light of these findings. The relationship of the location of the primary tear, blood pressure, and the age of the patient to prognosis is reviewed.