A case of left carotidocavernous fistula with right exophthalmos is reported. It is the thirteenth reported case in the literature since 1907. Mechanism of the exclusively contralateral exophthalmos is discussed. Temporary venous thrombosis of the ipsilateral ophthalmic vein and inferior petrosal sinus is likely. Successful treatment by detachable balloon with occlusion of the carotid siphon is described.