Screening and carotid endarterectomy have been advocated for asymptomatic carotid stenosis. However, the risk of stroke without treatment has not been adequately defined. We investigated the risk of stroke in the distribution of the asymptomatic carotid artery in 2295 patients randomised in the European Carotid Surgery Trial. During a mean follow-up of 4.5 years, there were 69 carotid territory strokes, nine of which were fatal, giving three year Kaplan-Meier risks of stroke and fatal stroke of 2.1% (95% Cl, 1.5-2.8) and 0.3% (95% Cl, 0.06-0.56) respectively. The stroke risk in the 127 patients with severe (70-99%) carotid stenosis was 5.7% (95% Cl, 1.5-9.8). Given these low stroke risks, the potential benefit of endarterectomy for asymptomatic carotid stenosis is small. Population screening is not justified and endarterectomy for asymptomatic carotid stenosis should only be performed in the context of well organised randomised controlled trials.