A 39-year-old woman was referred for assessment of abnormality of on a CT scan with a vascular anomaly of the aortic arch. This patient was completely asymptomatic with no concomitant pathologies and no reported prior trauma. Laboratory data for syphilitic or other microbial infections were negative. The diagnosis was confirmed by angiographic computed tomographic scan with 3-dimensional reconstruction. This technique documented the presence of the aneurysm and the left subclavian artery arising from the unique form of aneurysm. Early surgery was preferred because of the young age of the patient and the morphology and the size of the aneurysm (50 mm). Surgery was performed by a left postero-lateral thoracotomy through the forth intercostal space. Femoro-femoral partial cardiopulmonary bypass was used for distal perfusion. An aortic clamp was placed just distal to the left carotid artery, and a second clamp was placed in the descending thoracic aorta. The aortic isthmus was replaced with a 20-mm Dacron graft, and the left subclavian artery was reimplanted to the prosthesis with an 8-mm Dacron graft interposition. This aneurysm was the result of abnormal organogenesis of a primitive aortic arch and the remnant of the dorsal aorta, in other words, Kommerell's diverticulum. Microscopic examination demonstrated severe medial layer atrophy. In the light of the high risk of rupture, which was proved to be present by the very thin aneurysm wall at the time of surgery, we suggest early surgical treatment of idiopathic isthmus aneurysms in young patients regardless of aneurysm diameter.