In-stent restenosis (ISR) is a potentially preventable cause of stroke in carotid artery stent (CAS) patients. Understanding the frequency and timing of ISR would be useful in developing optimal protocols for carotid stent surveillance. The time course and frequency of moderate and severe ISR in our single institution prospective registry of CAS procedures is reported here. Data were collected prospectively from 296 consecutive elective CAS procedures. Doppler surveillance was performed at 1, 6 and 12 months and annually thereafter in some cases. Moderate ISR (>50%) was defined as a peak systolic velocity (PSV) > 200 cm/s. Severe ISR (>70%) was defined as PSV > 200 cm/s and end diastolic velocity > 125 cm/s or internal carotid artery/common carotid artery ratio >4. Patients with severe ISR underwent digital subtraction angiography for confirmation and possible retreatment. Clinical follow-up at 1 year was 98%. Clinical and ultrasound follow-up at 1 month was 100%, at 6 months 96% and at 1 year 91%. The incidence of all ISR at 6 months was 8%; 5% moderate and 3% severe. Two patients had asymptomatic occlusions at 6 months (0.8%). Patients with moderate ISR at 6 months did not progress to severe ISR. There were two strokes caused by stent thrombosis, one acute and one delayed. Doppler surveillance is important for detecting ISR after CAS procedures. Severe ISR (>70%) should be retreated in most cases as ISR may progress to occlusion. Asymptomatic patients with moderate ISR (50-69%) at 6 months can be safely followed conservatively. Progression of ISR after 6-12 months is uncommon over a 2-3 year period.