SUMMARY Seventy-three patients with an angiographically identified asymptomatic stenosis (>50%) and/or ulceration of the common carotid bifurcation have been followed from 6 months to 10 years (average 4 years). All patients had previously undergone contralateral carotid endarterectomy for a transient ischemic attack (TIA) or minor stroke in that carotid territory. During follow-up, 22 patients (30%) developed new symptoms of cerebral ischemia. Twelve developed ischemia referable to the previously asymptomatic side (10 TIA's, 2 strokes). Six developed recurrent ischemic symptoms in the territory of the previously symptomatic and operated carotid artery (2 TIA's, 4 strokes). Five developed ischemia in the vertebro-basilar territory (2 TIA's, 3 strokes). Thirteen patients (17%) died during follow-up, including 6 from cardiovascular disease and 3 from stroke.In our series the incidence of stroke in the territory of a significant asymptomatic carotid plaque was low (3%). Patients were as likely to develop stroke in the territory of a previously operated carotid artery (5%) or in the vertebro-basilar territory (4 %). It seems that a reasonable approach to the management of patients with asymptomatic carotid lesions is to keep them under review and to consider endarterectomy only if appropriate ischemic symptoms (which are most likely to be TIA's) develop.Stroke, Vol 13, No 4, 1982 PATIENTS WITH SIGNIFICANT but asymptomatic atheromatous plaques at the origin of the internal carotid artery are commonly identified in the investigation of symptomatic cerebrovascular disease by arterial angiography. Such patients are likely to be identified with increasing frequency as the non-invasive Doppler techniques become more reliable and as digital-subtraction angiography becomes more widely available. The proper management of such patients is uncertain as little is known of the natural history of asymptomatic carotid plaques. Some authors 1,2 advocate an aggressive approach, recommending prophylactic carotid endarterectomy in the belief that such lesions carry a greater risk of subsequent stroke without surgery. Others 3, 4 ' 5 have found little evidence that these lesions are particularly ominous and recommend a conservative approach, reserving the option of surgical treatment until such time as appropriate symptoms arise. We have carried out a retrospective survey of our patients with such lesions for the purpose of providing additional information regarding their natural history.Clinical Material Seventy-three patients with an asymptomatic atheromatous plaque at the common carotid bifurcation producing a significant lesion were available for study. For the purposes of this study, a significant lesion is defined as either a stenosis of the origin of the internal carotid artery of greater than 50%, an obvious ulceration, or both as determined angiographically. The 73 asymptomatic lesions were identified among a group of 324 patients who were investigated and treated by carotid endarterectomy for a symptomatic plaque