Background and Purpose Current methods of measuring carotid stenosis such as those used in the North American Symptomatic Carotid Endarterectomy Trial (NASCET) and European Carotid Surgery Trial (ECST) have limitations caused by difficulties in measuring the normal width of the distal internal carotid artery (ICA) or the carotid bulb. Methods We developed a new technique, the Carotid Stenosis Index (CSI), based on the known anatomic relationship between the common carotid artery (CCA) and ICA (1.2×CCA diameter=proximal ICA diameter). The normal ICA diameter can therefore be calculated from direct measurement of the CCA. Three blinded observers evaluated the angiograms of 57 patients (114 carotid arteries), previously screened with duplex ultrasonography, using the NASCET, ECST, and CSI methods. In a subset of 30 patients undergoing carotid endarterectomy, comparison was also made to computerized carotid plaque planimetry. Results The NASCET method could only be applied correctly in 89% and the ECST method in 95% of cases because of overlying vessels or inadequate views of the distal ICA or carotid bulb. An additional 9% of NASCET cases had a “negative” stenosis, in which the stenosis is wider than the distal ICA. The CSI method was applicable in 99% of cases. Interobserver comparison using ANOVA revealed significant differences using NASCET ( P <.0001) and ECST ( P <.001) but not CSI ( P =NS). NASCET had a sevenfold variation ( P <.01) and ECST a twofold variation ( P <.01) in results compared with CSI . The intraobserver reliability was 0.87 for NASCET, 0.86 for ECST, and 0.90 for CSI. However, the 95% confidence intervals for an individual measurement by an observer were ±30% for NASCET, ±19% for ECST, and ±15% for CSI. With linear methods of measurement there were significant differences between NASCET and CSI ( P <.0001) and ECST ( P <.0001) but not between CSI and ECST. A comparison of area derivations of these methods to carotid plaque planimetry revealed significant differences from NASCET ( P <.0001) but not ECST, CSI, or duplex methods. A CSI nomogram was created, allowing measurement of both linear and area percent stenosis. Conclusions CSI is the most reliable validated method of measuring carotid stenosis, and it correlates with duplex and carotid pathology.