The causal factors of aneurysm are not the same in the ascending and abdominal aorta. Atheroma is the dominant lesion in the abdominal aorta and genetic abnormalities predisposing to "mediacystic necrosis" are more frequently observed in ascending aortic aneurysms. The causal genetic abnormalities are multiple: abnormalities of fibrillin type I in Marfan's syndrome, collagen abnormalities in the Ehlers-Danlos syndrome, abnormalities of microfibrils, possible metallo-proteases, with many gene candidates. Finally, some ascending aortic aneurysms are associated with bicuspid aortic valves, some of which are familial. Familial factors are less evident in abdominal aortic aneurysms: genes favoring atheroma may be responsible but the factors which predispose to dilatation and non stenosis are not well known: they would seem to be general factors because the tendency to arterial dilatation is diffuse. In all cases, because of the high risk of disease in a close relative, a familial enquiry would seem to be justified with echocardiography for close relatives of affected subjects, especially when young and female.