The recently published clinical trials of carotid endarterectomy marked a turning point in carotid sonography, because they provided justification for seeking carotid stenosis with ultrasound in both symptomatic and asymptomatic patients. These trials also were a turning point because they set a new standard for measuring carotid stenosis from arteriograms, based on the comparison of the least diameter of the residual internal carotid artery (ICA) lumen and the diameter of the normal, distal ICA. The adoption of this new standard for arteriographic measurement has necessitated the redefinition of velocity criteria for duplex Doppler diagnosis of ICA stenosis. This article discusses the methods for establishing Doppler velocity criteria for the identification of clinically significant carotid ICA stenosis, based on the new standard for arteriographic measurement.