A family is described in which two brothers (44 and 48 years old) had aortic dissecting aneurysms (the first one died, the other one underwent surgical treatment and is still living) in the absence of clinical features of Marfan syndrome and of systemic hypertension. Two of the six living siblings have aortic dilation on echocardiography. Histologic examination of the aortic wall at autopsy or surgery revealed a loss of elastic fibers, deposition of mucopolysaccaride-like material and medionecrosis. We can postulate a genetically-determined disease of connective tissue usually described as "Marfan's forme fruste".