Occurrence of new symptoms of focal cerebrovascular disease, including completed cerebral infarction, transient cerebral ischemic attack and amaurosis fugax, was correlated with changes in size and morphology of atherosclerotic plaques at the carotid artery bifurcation visualized by real time B-mode ultrasonography. Bilateral serial images of 246 plaques in 123 patients were made in a time interval of 9 to 35 months between initial and follow-up studies. There was a significantly higher incidence of symptoms ipsilateral to plaques that grew [30 (25%) of 121] than of symptoms ipsilateral to plaques that diminished or remained unchanged [8 (8%) of 98] (p less than 0.001). Plaques that became hemodynamically obstructive by causing disruption of laminar flow or reduction of distal perfusion pressure at the ophthalmic artery were associated with a higher frequency of symptoms [12 (40%) of 30] than were nonobstructive plaques [26 (13%) of 201] (p less than 0.001). There was also a strong association between symptoms and plaque configuration. Mural plaques growing in a crescentic configuration along the wall of the carotid bifurcation between the first and second examination had a higher incidence of symptoms [22 (40%) of 56] than did plaques that grew but maintained a nodular configuration [8 (12%) of 65] (p less than 0.001), although the same proportion of each became hemodynamically obstructive. Sequential visualization of atherosclerotic plaques by real time B-mode ultrasonography of the carotid bifurcation may provide a method for studying the pathogenesis of arterial thrombosis.