This paper describes a prospective study comparing the preoperative Duplex ultrasound appearance of carotid bifurcation atheroma with the pathological characteristics found in the endarterectomy specimens of the same vessels. Initial studies of carotid atheroma using Duplex scanning classified plaques into heterogeneous and homogeneous and found a strong correlation between heterogeneous lesions and the presence of intraplaque haemorrhage or ulceration in the endarterectomy specimen. The B-mode classification of plaque appearance described in this paper is an expansion of the above classification. The study group comprised 220 patients who underwent 244 procedures. The indication for carotid endarterectomy was symptomatic disease in the great majority of cases. We found a high incidence of unstable plaque pathology in the operative specimens, and a predominance of the more echolucent ultrasound plaque appearances (types 1 and 2). There is a statistically significant relationship (p less than 0.001) between ultrasound appearance types 1 and 2 and the presence of either intraplaque haemorrhage or ulceration in the endarterectomy specimen.