PURPOSE: The purpose of this study was to determine the diagnostic value of a sign of early enhancing superior ophthalmic vein (SOV), as seen on helical CT images in patients with carotid-cavernous fistula (CCF). MATERIALS AND METHODS: This study involved 16 patients with CCF and 28 control patients. Axial CT images with scanning delays of 30 seconds following bolus injection of contrast material (90 mL, 3 mL/sec) were obtained, and this procedure was followed by coronal CT imaging. To determine the presence or absence of early enhancement or, dilatation of the SOV, bulging of the cavernous sinus, and enlargement of extraocular muscle, CT images were analysed by three observers in a blinded, random manner. Early enhancement of SOV was determined to be present where enhancement of the SOV was similar to or stronger than that of the ipsi-lateral posterior cerebral artery. RESULTS: A sign of early enhancing SOV was seen in 14 of the 16 patients with CCF but in no control patients (88% sensitivity and 100% specificity). The respective sensitivity and specificity of other CT features were 71% and 100% (dilatation of the SOV, as seen on axial images), 60% and 83% (dilatation of the SOV, as seen on coronal images), 71% and 89% (dilatation of the cavernous sinus), and 65% and 98% (enlargement of extraocular muscle). CONCLUSION: A sign of early enhancing SOV is a characteristic and specific CT finding of CCF, and is useful for the diagnosis of CCF.