Stenoses at the origin of 100 internal carotid arteries from patients with ischemic cerebrovascular disease were studied by intra-arterial angiography. Three principles were employed to evaluate the degree of stenosis: 1) verbal description, grading stenoses as mild, moderate of severe, 2) estimation of the true residual diameter, and 3) calculation of percent stenosis, defined here as the diameter of the artery at the origin relative to that at the level of the angle of the mandible. The true diameter, estimated to the nearest whole mm, corresponded well with the verbal description, and correlated significantly with percent stenosis. Since the true diameter is precisely defined, easy to estimate, and directly related to its hemodynamic effect, we suggest that this simple parameter, rather than the more commonly used, but more vaguely defined term "percent lumen reduction", is used to grade stenoses of the internal carotid artery.