Nineteen patients (20 operations) underwent elective carotid endarterectomy without arterial shunt. Carotid cross-clamping caused a significant decrease (from 61.2% to 49.5%, 19.1% decrease from the preclamp baseline) of the ipsilateral cerebral oxygen saturation and it increased to 65.6% after declamping. Cross-clamping also caused a significant decrease (from 2.9 Hz to 1.6 Hz) of the ipsilateral electroencephalogram main frequency and it increased to 3.6 Hz after declamping. Asymmetry of main frequency which was greater than 0.7 Hz was observed when that of oxygen saturation decreased more than 25% during cross-clamping. The reported data indicate that cerebral oxygen saturation less than 54-56.1% and its decrease more than 15.6-18.2% is found to be a predictor of neurologic compromise. In this study, the asymmetry of cerebral oxygen saturation more than 25% was also found to be a risk. Arterial shunt should be used in haemodynamically high risk cases.