On medical examination, the chest radiograph of a 25-year-old healthy woman incidentally revealed an abnormal shadow. Enhanced computed tomography (CT) revealed three saccular aneurysms, and the patient was referred for surgical treatment. Physical examination did not yield any particular findings. Three-dimensional CT showed that two of the aneurysms were located at the proximal descending aorta (cover). The larger aneurysm was 45 mm in diameter, and the smaller was 35 mm in diameter. The third aneurysm was located at the proximal part of the left subclavian artery, and it was 12 mm in diameter (cover). Thinning of the aorta and artery was also observed (A), but no other complications in the cardiovascular system were detected. Surgical treatment was performed. The aneurysms were exposed by a left thoracotomy via the fourth intercostal space (B). Graft replacement of the proximal descending aorta with reconstruction of the left subclavian artery was performed under deep hypothermic circulatory arrest. The blood vessels were very fragile and difficult to anastomose. Postoperative recovery was uneventful. The histopathological specimens of the resected aneurysms indicated severe cystic medial necrosis (elastic van Gieson stain, 40; C). Marfan syndrome and Ehlers-Danlos syndrome type IV (the vascular type) are well-known disorders characterized by the occurrence of aortic aneurysm or dissection at an early age. Molecular analyses of these syndromes have recently advanced. Mutations in the genes encoding fibrillin-1 and transforming growth factor beta receptor 1 and 2 have been shown in Marfan syndrome, and mutations in the gene encoding type III procollagen have been identified in Ehlers-Danlos syndrome type IV. In our patient, genetic testing for these genes was carried out, but it yielded negative results. Other cardiovascular disorders are expected to occur in this patient, and therefore, careful and lifelong follow-up is required.