We studied the carotid angiograms of 3007 patients in the European Carotid Surgery Trial. Poststenotic narrowing of the ICA was defined with use of the ratio of the lumen diameter of the ICA to that of the common carotid artery (CCA). The normal range of the ICA/CCA ratio was defined in 2966 symptomatic or contralateral carotid arteries with 0% to 49% stenosis. Arteries with 70% to 99% symptomatic stenosis and an ICA/CCA ratio below this range were categorized as narrowed