PURPOSE: MRI is known to be an effective imaging modality of the aorta and its role is steadily increasing in the evaluation of acquired aortic diseases including aortic dissections and aortic aneurysms. However, differentiation of the aortic dissections with thrombosed false lumen from the aortic aneurysm with mural thrombus in MRI has not been easy. Therefore, the authors tried to find the characteristic MR featrses which would to differentiate the two diseases. MATERIALS AND METHODS: MR images of 6 patients with thrombosed aortic dissection and 7 patients with thrombosed aortic aneurysms were reviewed retrospectively and compared in regand to shape and extent of thrombus, dimension of aorta, and luminal flow signal. RESULTS: Thrombosed aortic dissections showed sharply demarcated crescent shaped aortic wall thickening of even thickness involving long segment of the aorta, whereas thrombosed aortic aneurysms showed irregular aortic wall thickening of uneven thickness localized in the short dilated segment of the aorta. Characteristically aortic aneurysm with mural thrombus showed eccentric intraluminal slow flow signal. In contrast to the signal void of the true lumen in aortic dissections, the residual lumen of the aortic aneurysm with mural thrombus revealed intraluminal signal due to slow flow. CONCLUSION: Familiarity to these MRI findings of thrombosed aortic dissections and aortic aneurysms may lead to the accurate differential diagnosis in majority of cases.