Preoperative selective cerebral arteriography is carried out in order to determine if the circle of Willis is intact in the cases of the thoracic aortic aneurysms. As regard to the bilateral vertebrobasilar communications, 26 of 30 patients (86.7%) have good condition, 4 of 30 patients (13.3%) have poor, and in 2 cases the left vertebral artery supplied broad area of posterior cranial fossa. We believe that in the cases in whom the left vertebral artery is dominant, and cross collateral circulation is absent, perfusion of the left subclavian artery is necessary to prevent vertebrobasilar ischemia during aortic aneurysms operation under normothermia.