OBJECTIVE The value of carotid endarterectomy (CEA) depends on the safety of the operation. Transcranial Doppler ultrasound (TCD) was used to evaluate the possibilities of hypoperfusion, hyperperfusion, and embolization as causes of stroke and to evaluate the significance of Doppler microembolic signals (DMES). METHODS Five hundred CEAs were monitored with TCD of the ipsilateral middle cerebral artery during various phases of CEA to determine hemodynamic changes and incidence of DMES. Complications were graded according to their severity, and their probable cause was determined from TCD criteria and review of hospital charts. RESULTS We observed 24 cerebrovascular complications (4.8%), including 9 with transient ischemic attacks and 15 (3%) with permanent deficits. Among all cerebrovascular complications, embolism was judged to be responsible in 13 (54%; P < .02 compared with hypoperfusion), hyperperfusion in 7 (29%; P < .14 compared with hypoperfusion), and hypoperfusion in 4 (17%; P < .08 compared with embolism plus hyperperfusion). The surgeons responded to TCD information by several strategies depending on the TCD information. The incidence of permanent deficits diminished from 7% in the first 100 operations to 2% in the last 400 (P < or = .01). Shunting was more strongly associated with cerebrovascular complications than nonshunting, but this difference was not significant (P = .24). Intraoperative prevalence of DMES was strongly associated with cerebrovascular complications (P = .02). CONCLUSIONS Embolism is the principal cause of cerebrovascular complications from CEA; hyperperfusion and hypoperfusion are also important causes. TCD provides information that allows prompt identification and treatment of these three major causes of stroke from this operation. The perioperative stroke rate can be reduced by appropriate measures, taken by the surgeons, based on findings of TCD monitoring.