This study was designed to evaluate the possible contribution of 3-dimensional (3D) ultrasound (US) for noninvasive detection of extracranial carotid artery stenosis. Sixty-nine stenotic lesions of extracranial carotid artery were studied by (1) B-mode (Bm) US, (2) Doppler spectral analysis, and (3) a prototype of 3D vascular system. When indicated (46 stenotic lesions), biplane carotid angiography (CA) was performed. The degree of luminal narrowing measured as percent area reduction in the 3D data set correlated well with the degree of stenosis estimated by CA (r = 0.79, P <.01, mean difference 7.8% +/- 15.5%); however, for stenosis between 40% and 70%, 3D US tended to give higher values. Compared with CA, the sensitivity, specificity, diagnostic accuracy, and positive predictive value of 3D US for significant (> or =70%) stenosis were 96.0%, 77.7%, 88.3%, and 85.7%, respectively. Thus, 3D US showed good sensitivity and diagnostic accuracy for detection of significant stenosis of extracranial carotid artery. For stenosis between 40% and 70%, 3D US indicated a higher degree than CA; this finding suggests that CA may underestimate the severity of stenosis due to known discrepancies between linear measurement and true anatomic situation.