Using transcranial Doppler (TCD) ultrasonography in patients with high-grade (>=70%) internal carotid artery (ICA) stenosis, we examined the relation between the rate of TCD-detected silent microembolism of the ipsilateral middle cerebral artery and a history of recent (<121 days) ischemic symptoms attributable to the diseased ICA. In the so-defined neurologically symptomatic group (n = 33 patients), silent microembolic events occurred in 27 subjects (overall mean rate, 14/h ± 29). Among 56 neurologically asymptomatic patients matched for the degree of ICA stenosis, only nine showed such events (overall mean rate, 0.35/h ± 1.4). Across all 89 patients studied, an individual microembolic event rate ≥2/h had a positive predictive value of 0.88 for a history of recent symptoms. Our data suggest that TCD monitoring can provide reliable paraclinical evidence of “unstable ICA disease.”