A prospective study comparing the accuracy of B-mode ultrasound with evaluation of gross appearance in detecting plaque hemorrhage and ulceration is reported. Ultrasonography was performed on 78 vessels in 66 patients undergoing carotid endarterectomy. Carotid bifurcation atheroma were classified ultrasonographically as smooth, irregular, or ulcerated. Sonolucent areas were considered to be plaque hemorrhage. Analysis of carotid plaques removed at operation included macroscopic and microscopic findings. Duplex Doppler was only 44% sensitive but 78% specific for plaque hemorrhage. If all irregular atheroma were considered ulcerated, the sensitivity of Duplex Doppler was 81% with specificity only 33%. If only lesions with a clearly visualized crater were considered ulcerated, B-mode ultrasound was 85% specific but only 17% sensitive. False B-mode ultrasound diagnoses of ulceration were mainly due to culs-de-sac or pits in fibrotic plaque that looked like ulcers. Ulceration was not noticed on B-mode when there was a two-dimensional problem or calcification with shadowing. False B-mode diagnoses of plaque hemorrhage were due to atheromatous debris, ulcerated plaque hemorrhage, or calcification. Problems still exist in the B-mode ultrasound diagnosis of carotid plaque hemorrhage and ulceration.