A Curvilinear or thick band-like low density on aortic wall on CT scan is sometimes problematic. The point is whether its represents atheromatous plaque or thrombosed false lumen of the dissecting aneurysm. We reviewed 212 randomely selected chest CT cases with regard to low denisty on the thoracic aorta. Thir-ty two patients showed visible low density on the aorta. Seven patients had dissecting aneurysm confirmed by aortography and/or CT scan. Atheromatous palque did not exceed 4mm in its maximum thickness and appeared as curvilinear rim while thrombosed false lumen of the dissecting aneurysm exceeded 10mm in its maximum thickness in all cases with the shape of crescent half-moon or thick band. Maximum transverse diameter of thoracic aorta was more than 4cm in all cases of dissecting aneurysm while none of the atheromatous plaque cases showed more than 4cm. Thrombosed false lumen of the dissecting aneurysm can reliably be distinguished from atheroma by its thicker low density region and larger size of the aorta.