Background and Purpose - We sought to determine the incidence of recurrent carotid stenosis in patients in the Asymptomatic Carotid Atherosclerosis Study (ACAS) who had undergone carotid endarterectomy and were prospectively followed with Doppler ultrasound for up to 5 years. Methods - The ACAS database was interrogated to determine the rate of recurrent carotid stenosis (≤60%) based up angiogram-validated Doppler data, with a 90% and a 95% positive predictive value, as well as information concerning the technologists' interpretation of percent stenosis. These 3 parameters are reported for each of 3 time intervals: within 3 months of operation (residual disease), between 3 and 18 months (early restenoses), and between 18 and 60 months (late restenosis). Results - Of the 825 patients randomized to the surgical arm of the study, 720 actually underwent carotid endarterectomy, and 645 had complete ultrasound data. The aggregate incidence of residual and recurrent carotid stenosis for all time intervals ranged from 12.7% to 20.4%, depending on the positive predictive value confidence level desired. Residual disease occurred in 4.1% to 6.5%; true, early restenosis was found in 7.6% to 11.4%; and late restenosis occurred in 1.9% to 4.9%. None of the traditional risk factors showed a statistically significant effect on recurrent stenosis. The use of patch angioplasty closure reduced overall risk of restenosis from 21.2% to 7.1%, from 16.7% to 4.6%, and from 27.4% to 8.2%, depending on the PPV confidence level desired (P<0.001). Of the 136 patients judged to have recurrent stenosis, only 8 (5.9%) underwent reoperation (only 1 for symptoms). There was no correlation between late stroke and recurrent stenosis. Conclusions - Carotid endarterectomy is a durable procedure with a low rate of true restenosis, particularly when patch angioplasty is used to close the arteriotomy.