Spectral phonoangiography, a noninvasive method for measurement of the residual-lumen diameter of carotid stenosis by bruit analysis, was compared with x-ray angiography and direct measurement of the pathological specimen at carotid endarterectomy in 39 bifurcations from 36 patients. In six studies, the bruit was too faint to analyze. In 31 of the other 33 studies, the phonoangiogram predicted the residual-lumen diameter to within 0.5 mm of the measured value. Of the 39 contrast angiograms, 35 showed residual lumens within 0.5 mm of the value measured in the specimen, two showed lumens between 0.5 and 1 mm, and the sizes of two could not be estimated because of vessel overlap in all planes. Spectral phonoangiography and contrast angiography are both accurate methods for evaluation of carotid stenosis. Since phonoangiography is noninvasive, it may be of particular value in determining the natural history in patients with carotid bruits. (N Engl J Med. 1981; 305:417–9.) SINCE Fisher observed in 1951 that cerebral infarction could follow atherosclerotic stenosis of the carotid bifurcation,1 the detection of occlusive carotid disease has been of clinical interest. The advent of successful carotid surgery has heightened that interest and created a need for noninvasive assessment of the exact extent of carotid stenosis. Quantitative spectral phonoangiography is an accurate method for noninvasive assessment of the extent of carotid stenosis through spectral analysis of bruits arising from the carotid bifurcation.2 3 4 Agreement with radiographic assessment is excellent, even in the presence of transmitted heart murmurs.4 We studied 170 carotid bruits in 123 consecutive patients. . . © 1981, Massachusetts Medical Society. All rights reserved.