A persistent carotid-basilar anastomosis (primitive trigeminal artery), identified by four-vessel vertebral angiography, was shown to be the cause of a cavernous sinus fistula in a 51-year-old woman. The fistula, but not the primitive artery, was identified on a carotid arteriogram. Because of the flow contribution from the posterior circulation, balloon embolization via the carotid system failed, and the fistula was repaired through a direct surgical approach. The operative technique is described and the hemodynamic aspects of a cavernous sinus fistula that is related to this primitive anastomosis are reviewed.