Several clinical trials have demonstrated that carotid endarterectomy (CE) in symptomatic patients with 70-99% internal carotid artery (ICA) stenosis, when used appropriately in experienced surgical hands (postoperative complications of stroke and death must not exceed 7%) is safe and effective in preventing recurrence of ipsilateral carotid ischemia and, in particular, in preventing disabling ipsilateral stroke. Only five patients need to be treated to prevent one stroke in three years. The time of greatest risk of stroke after the development of symptoms was in the first six months, and the incremental risk decreased out to two years. Instead the risk of stroke with asymptomatic carotid stenosis is low. Forty-five percent of strokes in patients with asymptomatic 60% to 99% stenosis are attributable to lacunes or cardioembolism. Because CE cannot prevent stroke of cardioembolic origin and is less likely to prevent stroke of lacunar origin, it is doubtful that CE can be justified for most patients with asymptomatic arteries.