Marfan's syndrome associated with multiple arterial aneurysms of the abodominal aortic branches in a pair of identical twins is described. The first case, a 21-year-old male, visited our affiliated hospital because of severe upper abdominal pain. Computed tomography (CT) and aortography (AOG) demonstrated multiple abdominal aneurysms, i.e., celiac artery, bilateral renal arteries and superior mesenteric artery, but there were no findings of dissecting aneurysm of the aorta. The aneurysm of the superior mesenteric artery, which had ruptured, was replaced with a greater saphenous vein graft. The postoperative course was uneventful until sudden cardiac arrest occurred on the 5th POD. An autopsy revealed a dissecting aortic aneurysm (DeBakey type I), massive intrathoracic hemorrhage, and subarachnoidal hemorrhage. Two years later, the second case, the 23-year-old twin brother of the prior case, visited our hospital because of upper abdominal pain similar to that of his brother. The clinical course bore a close resemblance to his brother's. CT and AOG revealed the intact aorta and aneurysms of the celiac artery and right renal artery. The ruptured aneurysm of the celiac artery was ligated. Immediately after his returning to the ICU, he developed sudden massive intrathoracic bleeding and cardiac arrest. This is the fourth report of identical twins with Marfan's syndrome in Japan. It is also noteworthy that in both cases rupture of the extra-aortic aneurysm occurred prior to the aortic dissection or aneurysmal formation.