We have developed a scoring system to quantify extent of extracranial carotid artery atherosclerosis using real-time ultrasound (B-mode). To evaluate repeatability of this scoring system we correlated repeat scores obtained within a short interval of one another (6 months) in 52 individuals. We compared repeatability of extent measurements with repeatability of a measure of severity (single most severe lesion). Correlations between first and second studies for severity were weak (r2 = 0.20) but significant (p less than 0.001). Extent scores correlated much better (r2 = 0.77, p less than 0.001). In another group of 22 patients we found that the extent of atherosclerosis decreased following endarterectomy. We used this method to determine changes in extent of carotid atherosclerosis with age in two sets of individuals. One consisted of a cohort of 22 patients who underwent repeat B-mode studies separated by 1 1/2-3 years. This cohort demonstrated an increase in carotid score with age (p less than 0.05). In a second group of volunteers undergoing cardiac catheterization and B-mode evaluation of the carotid system, carotid scores could be compared in individuals with age differences that averaged 15 years. Extent of carotid atherosclerosis was significantly greater in older individuals (p less than 0.01) and differences in extent with age were exaggerated in patients with coronary disease compared to coronary disease free controls.