The clinical manifestations of carotid cavernous fistula, an abnormal arteriovenous connection between the cavernous sinus and the carotid artery, can closely mimic the cardinal signs of Graves' ophthalmopathy, an inflammatory disorder of the orbit usually associated with autoimmune thyroid disease. Therefore, carotid cavernous fistulas are generally considered in the differential diagnosis of Graves' ophthalmopathy, especially when the eye involvement is unilateral or asymmetric, and there is the need for exclusion of rarer etiologies of orbital disease. This is the first report of the simultaneous occurrence of Graves' ophthalmopathy and carotid cavernous fistula. The patient was a 67-yr-old woman who presented with a history of Graves' disease with mild bilateral ophthalmopathy treated with radioiodine following a 10-yr therapy with methimazole; after radioiodine treatment, ophthalmopathy deteriorated. At the time of our initial clinical evaluation the ocular involvement of the patient was symmetric, and no evidence of any associated condition was found. However, the response of eye disease to corticosteroid treatment was markedly unequal, resulting in evident asymmetry. This prompted a reconsideration of the diagnosis and a new evaluation of the patient with sensitive techniques, leading to the further diagnosis of carotid cavernous fistula.