To evaluate the effect of carotid surgery on the cerebral circulation, transcranial Doppler sonography and ocular pneumoplethysmography were performed on 36 patients who underwent unilateral carotid endarterectomy. Ocular pneumoplethysmography and transcranial Doppler sonography tests were performed within a week before and after operation, and transcranial Doppler sonography was repeated greater than or equal to 30 days after operation. Middle and anterior cerebral arteries were insonated bilaterally, and flow velocities ipsilateral (iMCAFV or iACAFV) and contralateral (cMCAFV or cACAFV) to the side of surgery were recorded. The iMCAFV and iACAFV increased significantly in the immediate postoperative period, and the iMCAFV remained elevated on the second follow-up study. Patients with greater than or equal to 75% ipsilateral carotid stenosis (N = 23) had increased iMCAFV, iACAFV, and decreased cACAFV after operation, whereas those with less than 75% stenosis (N = 13) had no significant transcranial Doppler sonography changes. Those with greater than or equal to 75% contralateral carotid stenosis (N = 17) had significant increases in iMCAFV, cMCAFV, and iACAFV after operation, whereas those with less than 75% contralateral carotid stenosis had no significant transcranial Doppler sonography changes. A subset of patients (N = 13) did not increase iMCAFV after surgery. The ocular pneumoplethysmography changes were significantly different in both groups when preoperative and postoperative values were compared. We conclude that carotid endarterectomy can cause lasting cerebral hemodynamic changes, but that its effects are not uniform among all patients.