The clinical, aortographic and pathologic features of six patients with acute retrograde dissection of the ascending thoracic aorta are presented and appropriate guidelines for surgical treatment are proposed. Although acute dissection of the aorta was the clinical diagnosis in these patients, clinical features were not helpful in identifying the pattern or extent of dissection. Aortography demonstrated that the site or sites of intimal tear occurred adjacent to the origin of the innominate artery (three patients) or left subclavian artery (two patients); an intimal tear was not seen in one patient. Five of the six patients also had acute or chronic (or both) antegrade dissection of the descending thoracic aorta. Retrograde dissection of the ascending aorta occurred in the outer portion of the media. The primary histologic changes were fragmentation of elastic fibers and fibrosis of the media. Three patients died, including two of the five patients who were treated surgically; the other three patients survived. Immediate replacement of the ascending aorta with a Dacron graft is recommended to prevent lethal complications due to extension of retrograde dissection, even though dissection may involve both the ascending and descending aorta and the site of intimai disruption may not be resected. The aortic arch or descending thoracic aorta, or both, can be replaced later, utilizing total body hypothermia and temporary circulatory arrest, if further dis-section or enlargement of the aorta occurs after emergency operation on the ascending aorta.