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To examine duplex ultrasound (US) criteria for carotid in-stent restenosis (ISR).Carotid artery stent (CAS) placement is an alternative to surgery for the treatment of carotid stenosis in high surgical risk patients. US is the primary method used to follow carotid stent patency. This study investigates US velocity measurements in carotid ISR.Two hundred sixty consecutive patients with CAS placement from June 2000 to June 2004 were followed with serial US. ISR was determined by using the standard US velocity criteria for nonstented carotid artery using peak systolic velocity (PSV), end-diastolic velocity (EDV), and internal carotid artery to common carotid velocity ratio (ICA/CCA ratio). Patients suspected of having carotid ISR ≥50% by US, underwent invasive angiography with stenosis graded by NASCET criteria. Results were compared to patients with nonstented carotid artery stenosis using Two-tailed Student'st-test.PSV and ICA/CCA ratio increased to a greater degree in ISR. In 50–69% stenotic arteries, the mean ICA/CCA ratio was 2.76 ± 0.7 in the ISR group compared to 2.04 ± 0.3 in the nonstented carotid group (P< 0.05). In ≥70% stenotic arteries, there were increases in PSV (520 ± 93 vs. 362 ± 60,P< 0.05) and ICA/CCA ratio (7.58 ± 2 vs. 4.51 ± 1.3,P< 0.05) in ISR versus nonstented carotid arteries, respectively.PSV and ICA/CCA ratio in ISR increased to a greater extent for angiographic stenosis ≥50%. PSV 240 cm/sec and ICA/CCA ratio 2.45 are optimal thresholds for ≥50% ISR, and PSV 450 cm/sec and ICA/CCA ratio 4.3 are optimal thresholds for ≥70% ISR.