Stroke is the fourth leading cause of death in the United States.¹ Each year nearly 800 000 people have a stroke, 600 000 for the first time.¹ Atherosclerotic narrowing of the internal carotid arteries—carotid artery stenosis (CAS)—is an important cause of stroke.² Ninety percent of all strokes are ischemic¹ and, of these, 10% are caused by large artery stenosis.³ CAS is initially asymptomatic, with its first manifestation frequently transient ischemic attack or stroke.² A meta-analysis of international studies suggested a prevalence of asymptomatic CAS of at least 50% (moderate stenosis) in 4.2% of the general population, increasing with age to 7% of women and 12% of men older than 70 years.⁴ However, most studies suggest a higher 5-year risk of carotid territory ischemic stroke in individuals with severe stenosis (≥70% narrowing), a degree present in only 0.5% to 1% of US cohorts.³