The various pathological types of aneurysm are reviewed and their natural history discussed in relation to morphology, etiology and site of the aneurysm. For diagnostic purposes, computed tomography is a reliable method in the detection of aortic aneurysm; for evaluation of the ascending segment, ultrasonography may also be considered a useful procedure. The two non-invasive methods are particularly valuable in monitoring patients with thoracic aneurysms not (yet) suitable for surgery. In the light of the literature and our own experience, practical guidelines for adequate treatment of non-dissecting aneurysms of the thoracic aorta are elaborated with respect to aneurysm site. In patients with anulo-aortic ectasia, surgical treatment is clearly indicated in the presence of typical Marfan syndrome and in cases showing a demonstrable increase of aneurysm dilatation and/or of aortic regurgitation (hospital mortality 5% in our experience); irrespective of these criteria, surgery should be considered if the echocardiographic root diameter is greater than 6 cm. Replacement of the aortic arch carries a high surgical risk and is suggested for patients suffering from symptoms such as pain or compression of adjacent tissues; in asymptomatic patients, arch resection is scarcely advisable unless the aneurysm is of luetic origin. For aneurysms of the descending aorta, the operative indication will also be related to clinical symptoms as well as to aneurysm expansion and growth; in asymptomatic subjects, surgery is recommended in the presence of a circumscribed posttraumatic false aneurysm, the operative risk being very low in this patient group. In cases with suspected aortic dissection, computed tomography is considered to be the primary diagnostic procedure. Angiography may be mandatory only for those few presurgical candidates who require accurate assessment of aortic valvular regurgitation and determination of peripheral organ perfusion. The extremely unfavourable natural history of acute aortic dissection (50% survival after 48 hours following onset of symptoms) clearly calls for immediate operative treatment in high-risk patients, that is in subjects with acute ascending aortic dissection (hospital mortality 28% in our series). The better prognosis of descending aortic dissection suggests that in these cases conservative hypotensive therapy is the treatment of choice; surgical intervention in type B acute dissection is indicated only if occlusion of a major aortic branch occurs or if impending rupture of the dissecting hematoma becomes evident.(ABSTRACT TRUNCATED AT 400 WORDS)