A young woman presented with a spontaneous carotid-cavernous fistula. She also had fibromuscular dysplasia of the extracranial cerebral arteries. The possible relationship of the two diseases brought up important questions concerning how to manage the patient. Treatment by obliteration of the carotid circulation on one side would be expected to increase blood flow on the other side. If the cavernous carotid artery on the second side was weakened by fibromuscular dysplasia, the increased flow might predispose to the formation of a second carotid-cavernous fistula. We first attempted to avoid surgery. When progressive symptoms occurred, the cavernous sinus was obliterated with bronze wire, thus preventing venous shunting.