To evaluate the frequency of subclinical cerebral complications of carotid endarterectomy, 19 patients were studied before and 1 month after surgery. Significant neurological complications were detected only in 1 patient (5%), while neuropsychological deterioration was found in 12 patients (63%) as compared to 5 (29%) of 17 control subjects undergoing other vascular surgery procedures (p < 0.05). MRI detected new cerebral lesions in 3 (16%) patients, who also showed a decline in cerebral perfusion reserve in the peri-infarct area as assessed by acetazolamide-enhanced SPET. Global cerebral perfusion reserve declined in three patients. Quantitative EEG did not reveal deterioration in any patients. The possible prognostic significance of these sublinical complications should be defined.