The authors report a technique to precisely localize a fistulous opening in the carotid artery. The patient is heparinized and a Prolo catheter is introduced into the internal carotid artery and inflated distal to the approximate site of the fistula. Heparinization allows the balloon to be inflated long enough to obtain and analyze high-quality angiography film without fear of thromboembolism generated by the temporary balloon occlusion. Contrast material injected through the Prolo catheter proximal to the balloon reveals a small segment of cavernous carotid artery between the inflated balloon distally and the fistula proximally. The venous structures are now only faintly opacified and cannot obscure the morbid anatomy of the exact fistulous tear in the carotid artery. If the balloon is placed exactly opposite to the site of the fistula, a standing, stagnant column of dye forms a cast of the cavernous, petrous, and cervical carotid artery. Once the fistula is localized with this method, it may be obliterated by any therapeutic means preferred. If the Prolo catheter is used for intraluminal occlusion, then a transfemoral contralateral carotid angiogram is done before the heparin is reversed to confirm that the balloon has not been placed proximal to the fistula.