We studied 110 carotid arteries of 55 patients with unilateral or bilateral carotid stenosis diagnosed with selective angiography, by using Transcranial Doppler to detect high intensity transient signals (HITS) in the middle cerebral arteries (MCAs). HITS identified as embolic signals were prevalent (P < 0.05) in the MCAs on the same side as severe (70-99%) stenosis (22 of 51 = 43.1%) compared to moderate (30-69%) stenosis (5 of 37 = 13.5%). No HITS were observed in the MCA on the same side as normal control carotid arteries (n = 17) [occluded arteries (n = 5) were not considered]. HITS were more prevalent (P < 0.05) in the MCAs on the same side as ulcerated plaques (14 of 23 = 60.9%) compared to non-ulcerated plaques (13 of 65 = 20%), and all moderate stenoses producing HITS presented ulceration of the plaque. Ulcerated plaque groups showed a higher mean number of HITS than non-ulcerated plaque groups and no significant difference was noted between moderate and severe stenosis, between superficial or deep ulcerations and between ulcerations with flap or without flap. Therefore, severe carotid stenosis and moderate stenosis with plaque ulceration result in angiographic findings most frequently associated with HITS. Further studies are necessary to evaluate the clinical significance of this finding.