From October, 1969 to December, 1982, 1117 aortic valve replacements were performed at the Foch Medico-Surgical Centre. Five of these operations (0.4%) were followed, within 6 months to 6 years (mean 37 months), by acute dissection of the whole ascending aorta with a varying degree of extension further down. Such accidents raise problems of aetiology, diagnosis, treatment and possibly prevention. All 5 patients had an emergency operation with prosthetic replacement of the ascending aorta; in some cases the 2 cylinders were glued to the distal and proximal aortic stumps. The other forms of treatment were related to the lesions encountered. All patients survived and underwent follow-up angiography after 6 months to 5 1/2 years. Lesions of cystic necrosis of the media were regularly found at histology. In every case during the first operation the surgeon reported that the aorta was dilated or particularly fragile. The question therefore arises as to whether ascending aortas that are fragile and of more than 5 cm in diameter should be systematically replaced.