The correlation between the computerised measurement of carotid plaque echogenicity on high resolution ultrasound imaging and plaque histology was studied in 52 patients undergoing carotid endarterectomy. Thirteen plaques were from asymptomatic patients, 15 were associated with amaurosis fugax, 10 with transient ischaemic attacks and 14 with stroke. Longitudinal images of the anterior and posterior component of each plaque were obtained by ATL Ultramark-4 Duplex scanner and were transferred to a computer. Using an image analysis program the median of the overall grey scale content of each plaque component was evaluated and used as a measure of echogenicity. Following carotid endarterectomy each plaque specimen was divided into anterior and posterior component and then fixed, oriented, sectioned and stained in the longitudinal plane corresponding to the ultrasound image. Plaque histology sections were then examined by computer morphometric analysis and the percentage surface areas of fibrous tissue, lipid deposits and haemorrhage were calculated. This was then correlated with the grey scale median for each plaque component. Plaques with a high lipid and haemorrhage content as established histologically had a low grey scale median (Spearman correlation r=-0.351, p<0.05) and those with a high fibrous content had a high grey scale median (r=0.411, p<0.001). This study has shown that computerised measurement of carotid plaque echogenicity correlates well with histology and could be used to predict plaque composition, thus identifying high risk plaques with high lipid and haemorrhage content.