Massive thrombi are sometimes present in aortic aneurysms, which cause severe complication such as distal arterial thromboembolism, and greatly influence the prognosis. Such thrombi can be easily detected by CT scan and ultrasound. However these imaging techniques can only demonstrate the presence of thrombi, and are not able to indicate these activity. We performed ¹¹¹In-labeled-platelet scintigraphy (platelet scinti.) in 27 cases of aortic aneurysms (13 true aneurysms and 14 dissecting aneurysms) and 13 postoperative cases of dissecting aneurysms, and compared the findings of CT scan. In some cases, the findings of platelet scinti. were markedly different from the findings CT scan. And our results suggested that the radioisotope deposit revealed by platelet scinti. was reflected thrombotic activity. In one case of dissecting aneurysm, marked RI deposit was revealed by platelet scinti., and subsequently the false lumen was occluded. One postoperative case of dissecting aneurysm showed marked RI deposit and, distal arterial thromboembolism developed. ¹¹¹In-labeled-platelet scintigraphy is thought to be useful to estimate thrombotic activity in aortic aneurysm, and to predict complications and the prognosis.