In a 16-year-old boy, minor symptoms of an arteriovenous shunt into the cavernous sinus recurred 3 months after the performance of a trapping operation of the internal carotid artery on the side of a traumatic carotid-cavernous sinus fistula. An arterial and venous cerebrovascular Doppler examination demonstrated increased blood flow in the periorbital veins of both sides, with a pathological flow direction, i.e., from intra- to extracranial. Furthermore, an internal-like artery was found at the level of the carotid bifurcation on the side of the trapped internal carotid artery, and increased blood flow was registered in the homolateral vertebral artery. Angiography confirmed occlusion of the internal carotid artery 0.5 cm distal to the carotid bifurcation, but showed blood flow from the homolateral vertebral artery through a persistent 3rd cervical intersegmental artery into the trapped portion of the internal carotid artery. The latter fed blood into the cavernous sinus through the still-existing arteriovenous fistula. Insufficient involution of the 3rd cervical intersegmental artery was assumed. Obviously, this collateral artery dilated in the presence of a major blood pressure gradient between the vertebral artery and the trapped segment of the internal carotid artery. Thus, a persistent cervical intersegmental artery can be a cause for a recurrent carotid-cavernous sinus fistula.