Noninvasive diagnostic studies (oculoplethysmography, pulsed Doppler arteriography, and phonoangiography) were used to follow the postoperative courses of 172 patients who had 199 carotid endarterectomies. There were 24 restenotic arteries in 21 patients who underwent 29 operations. Fifteen restenotic lesions in 14 patients were detected solely by noninvasive testing. These patients are being observed closely and remain asymptomatic. One has been operated on for progression of disease. Either transient or permanent neurologic deficits developed in nine as the initial indication of recurrent stenosis or occlusion; three of these subsequently have undergone reoperation. Patients with bilateral disease are at increased risk of restenosis. Routine testing of all patients undergoing carotid endarterectomy is recommended 1, 3, and 12 months postoperatively to detect and observe stenosis on both the side operated on and the contralateral side before clinical symptoms develop.