Objective: The aim of this study was to investigate a possible overestimation of the degree of contralateral carotid artery stenosis by duplex in patients with significant bilateral carotid stenoses who are to undergo carotid endarterectomy (CEA). Methods: A retrospective analysis was performed of all patients undergoing CEA in our center over a period of 11 years. Pre- and postoperative duplex ultrasonography measurements of peak systolic velocity (PSV) and end diastolic velocity (EDV) were compared and used to classify the degree of stenosis. Univariate analysis was performed to indicate possible predictors for contralateral stenosis overestimation. Results: A total of 384 CEA procedures in 357 patients were performed in our hospital. Pre- and postoperative bilateral duplex measurements were available in 135 patients. Forty-four out of 135 patients (33%) were preoperatively identified as having significant stenosis (>60%) of the internal carotid artery on both sides. In these patients, postoperative duplex measurements of the contralateral carotid showed a decrease in mean (SD) PSV from 2.53 (1.11) m s-1 to 1.97 (0.87) m s-1 (P < 0.01) and a decrease in EDV from 0.87 (0.60) m s-1 to 0.60 (0.36) m s-1 (P < 0.01). The absolute changes in contralateral PSV and EDV after CEA were larger among patients with a higher degree of stenosis preoperatively. These changes led to reclassification of stenosis to a lesser degree in 24 (55%) patients. In 16 cases (36%), this resulted in a measured stenosis on the contralateral side of less than 60%. Conclusions: One-third of the patients with duplex measurements consistent with bilateral significant carotid stenosis did not have a significant contralateral stenosis by duplex after CEA. Therefore, additional postoperative duplex measurement is advisable before planning contralateral CEA. © 2009 Elsevier Ireland Ltd.