124 carotid arteries of 62 patients with unilateral or bilateral carotid stenosis proven by selective angiography were investigated by transcranial Doppler to detect high-intensity transient signals (HITS) in the middle cerebral arteries (MCAs). HITS identified as embolic signals were detected in 29 of 124 (23.3%) MCAs and in all cases were asymptomatic. HITS were more common in the MCAs on the same side as high-degree (> 70%) carotid stenoses (24 of 57 = 42.2%) compared with low-degree (< 70%) carotid stenoses (5 of 28 = 17.9%, p < 0.05). No HITS were detected in the MCAs on the same side as normal (24) and occluded (15) carotid arteries. About the clinical features related to the stenoses, HITS were detected with a nonsignificant prevalence in the MCAs on the same side as symptomatic stenoses (16 of 43 = 37.2%) compared with asymptomatic stenoses (13 of 42 = 30.9%) and a relationship between HITS number and time elapsed from symptoms was observed. All symptomatic carotid stenoses > 70% (33) underwent endarterectomy and none of them showed HITS after surgical treatment. These results encourage the feasibility of prognostic studies to evaluate the clinical significance of embolic signals and suggest that silent microembolism could be helpful in selecting a high-risk group of asymptomatic or < 70% carotid stenoses for endarterectomy.