Duplex scanning and spectral analysis were used to detect carotid artery stenosis following 89 endarterectomies in 76 patients with a mean postoperative follow-up interval of 16 months. Operative arteriography was used in all cases to verify the technical result. Spectral changes indicating greater than 50%-diameter internal carotid stenosis were observed postoperatively in 32 of the 89 sides. Serial follow-up of 22 stenotic sides showed persistent stenosis in 12, regression of stenosis in nine, and internal carotid occlusion in one. The estimated overall incidence of persistent high-grade stenosis was 19%. Recurrent neurologic symptoms occurred in eight patients. This incidence of postoperative carotid stenosis is higher than estimates based on clinical criteria. The transient nature of some early postoperative stenosis is consistent with proliferation and regression of myointimal lesions in response to arterial injury.