In 23 patients carotid endarterectomy was performed in the presence of occlusion of the contralateral carotid artery. Postoperative morbidity and mortality were nil. Probably the low rate of complication was due to careful attention to preserving cerebral circulation during the operative procedure and to delaying operation until the patient was recovering from an acute cerebral infarction. An exception to the latter policy was made in two patients with an evolving stroke unresponsive to anticoagulants. Of the 23 patients, three have subsequently died, and two have sustained a further episode of cerebrovascular insufficiency. One of these episodes may have been due to a cerebral embolus originating in the heart.