Six consecutive cases with spontaneous carotid-cavernous fistulas were examined with orbital phlebography and bilateral carotid angiography. One of the fistulas was fed only by one internal carotid artery, 1 only by the accessory meningeal branch of the maxillary artery, while 4 were supplied by external as well as internal carotid arteries. The findings at phlebography seem to be characteristic. Thrombosis was demonstrated in all 6 cases. Different mechanisms for the genesis of thrombosis in association with carotid-cavernous fistulas are proposed. Alternative therapy methods are discussed.