Carotid percutaneous transluminal angioplasty/stenting has become an accepted treatment modality for carotid artery stenosis in high-risk patients. There has been an ongoing debate regarding which duplex ultrasound (DUS) criteria to use to determine the rate of in-stent restenosis. This prospective study revisits DUS criteria for determining the rate of in-stent restenosis. In analyzing a subset of 12 patients (pilot study) who had both completion carotid angiography and DUS within 30 days, 10 patients with normal post-stenting carotid angiography (< 30% residual stenosis) had peak systolic velocities (PSVs) of the stented internal carotid artery (ICA) of < or = 155 cm/s and two patients with > or = 30% residual stenosis had internal carotid artery (ICA) PSVs of > 155 cm/s. Eighty-three patients who underwent carotid stenting as part of clinical trials were analyzed. All patients underwent post-stenting carotid DUS that was done at 1 month and every 6 months thereafter. PSVs and end-diastolic velocities of the ICA and common carotid artery were recorded. Patients with PSVs of the ICA of > 140 cm/s underwent carotid computed tomographic (CT) angiography. The perioperative stroke rate was 1.2%. When the old DUS velocity criteria for nonstented carotid arteries were applied, 54% of patients had > or = 30% restenosis (PSV of > 120 cm/s), but when our new proposed DUS velocity criteria for stented arteries were applied (PSV of > 155 cm/s), 33% had > or = 30% restenosis at a mean follow-up of 18 months (p = .007). The mean PSVs for patients with normal stented carotid arteries based on CT angiography, were 122 cm/s versus 243 cm/s for > or = 30% restenosis and 113 cm/s versus 230 cm/s for > or = 30% restenosis based on our new criteria. The mean PSVs of in-stent restenosis of 30 to < 50%, 50 to < 70%, and 70 to 99%, based on CT angiography, were 205 cm/s, 264 cm/s, and 435 cm/s, respectively. Receiver operating curve analysis demonstrated that an ICA PSV of > 155 cm/s was optimal for detecting > or = 30% in-stent restenosis, with a sensitivity of 100%, a specificity of 90%, a positive predictive value of 74%, and a negative predictive value of 100%. The currently used carotid DUS velocity criteria overestimated the incidence of in-stent restenosis. We propose new velocity criteria for the ICA PSV of > 155 cm/s to define > or = 30% in-stent restenosis.