PURPOSE: The purpose of this study was to evaluate the CT and MR features of aortic arch aneurysms and todetermine the differences between involved segments and morphologic types according to their causes. MATERIALS AND METHODS: Twenty-nine patients with aortic arch aneurysms who underwent CT scanning(n=24) and/or MR imaging(n=16)were retrospectively evaluated. The aneurysms were analyzed with respect to location of involved segment,morphology, direction and size, and morphologic differences between aneurysms were compared according to causes. RESULTS: The causes of arch aneurysms were atherosclerosis in 25 patients(86%), trauma in three (10%) and infection in one (4%). Arch aneurysms were frequently located at the arch only(n=17,59%), ascending aorta toarch(n=6,21%), arch to descending aorta(n=4,14%), or ascending aorta to descending aorta(n=2,7%). The shape of theaneurysm was fusiform in 15 patients and saccular in 14. Atherosclerotic aneurysms(n=25) were fusiform in 15patients and saccular in ten. Arch aneurysms due to trauma and infection(n=4) were saccular. MRI was more helpfulthan CT scanning involved site, direction, and morphology of the aneurysm. CONCLUSION: Bothe CT scanning and MRIeasily diagnose arch aneurysms, though MRI is a very useful imaging modality for evaluating involved aorticsegments and morphologic types. Aortic arch aneurysms are either fusiform or saccular. Most saccular aneurysmsinvolve the aortic arch, whereas the involvement of fusiform aneurysms is more varied. Atherosclerosis is the mostcommon cause of both fusiform and saccular arch aneurysms.