Background and Purpose-In patients with both symptomatic and asymptomatic carotid artery stenoses, the relationship between carotid plaque characteristics and transcranial Doppler (TCD)-detected microembolic signals (MES) is unclear. The purpose of this study was to examine the relationship between macroscopically described plaque characteristics and MES in patients undergoing carotid endarterectomy. Methods-Sequential patients scheduled for carotid endarterectomy underwent preoperative 30-minute TCD monitoring of the ipsilateral middle cerebral artery to detect MES. TCD signal analysis, by researchers who were blinded to patient information, was performed offline. Clinical variables of patients and macroscopic carotid plaque features seen at surgery were documented prospectively. Results-Of the 109 patients (74 male, 35 female; mean age, 68.8Ϯ8.7 years) enrolled, 71 had ipsilateral carotid territory symptoms. MES were detected in 27 of all patients (25%). Twenty-two of 71 symptomatic patients (31%) compared with 5 of 38 asymptomatic patients (13%) had MES (Pϭ0.046). Also, symptomatic patients had more emboli (total MES counts) than asymptomatic patients (Pϭ0.010). The presence or absence of MES was not associated with plaque characteristics. Conclusions-Our data do not confirm previous reports of an association between MES and macroscopic plaque characteristics. We hypothesize that smaller platelet aggregates and fibrin clots, which are not detected macroscopically, are the most likely sources of TCD-detected MES. Key Words: carotid endarterectomy Ⅲ carotid stenosis Ⅲ middle cerebral artery Ⅲ ultrasonography, Doppler, transcranial C arotid stenosis is an important cause of ischemic stroke, with artery-to-artery embolism being the most common mechanism. In 1991, the European Carotid Surgery Trial (ECST) and North American Symptomatic Carotid Endarterectomy Trial (NASCET) investigators reported a beneficial effect in favor of carotid endarterectomy (CEA) in recently symptomatic patients with high-grade carotid artery stenosis. 1,2 The reduction in stroke risk is attributed to removal of the cerebral embolic source in most cases.Asymptomatic cerebral microembolic signals (MES) can be detected in patients with carotid stenosis through the use of transcranial Doppler (TCD). 3,4 Several observations suggest that MES detected distal to a high-grade carotid stenosis are due to unstable plaque. MES are more common among patients with recent carotid artery stroke and are more commonly detected among patients with more severe carotid artery narrowing. [5][6][7][8] Moreover, CEA tends to result in a rapid decline of MES, 9,10 suggesting that their temporal source has been removed.The frequency with which MES can be detected may depend on a number of factors such as duration of monitoring and the interval between symptom onset and TCD study. Reported incidence varies from Ϸ12% 11 to 100%. 9 MES may also be related to carotid plaque morphology. Valton et al 12 showed that MES were associated with carotid plaque ulceration on...