Of 185 patients who consecutively underwent carotid endarterectomy five years ago, 135 had a patent asymptomatic contralateral internal carotid artery (ICA). During follow-up (median, 59 months), 36 patients developed new neurologic symptoms (18 strokes and 18 transient ischemic attacks). Thirteen patients developed symptoms referable to the territory of the previously asymptomatic ICA (five strokes and eight transient ischemic attacks). Using life-table analysis, the annual stroke rate was estimated to be 1% and 2.2% considering the previously asymptomatic and symptomatic ICA, respectively. Separating patients according to the degree of stenosis on the preoperative angiogram and according to the presence of ulceration revealed a significantly higher incidence of neurologic events and strokes in patients with stenoses exceeding 50% and/or patients with obvious ulcerations. Although the risk of stroke without warning was increased in these subgroups, we did not consider the risk high enough to warrant prophylactic endarterectomy. An exception enough to warrant prophylactic endarterectomy. An exception may be the patient with a more than 90% stenosis.