Etiologic importance of the intimal flap of the external carotid artery in the development of postcarotid endarterectomy stroke. A technically unsatisfactory end point (transition from the removed diseased plaque to normal distal intima) leading to an intimal flap of the external carotid artery has been identified as a source of perioperative stroke. The mechanism involves thrombus formation with retrograde propagation of the thrombus and subsequent embolization of the internal carotid artery. This report describes three cases illustrating this mechanism and methods of identification and correction. This mechanism of postoperative stroke adds further justification for the routine use of intraoperative surveillance studies to document the technical result of endarterectomy involving the internal and external carotid arteries. When an unsatisfactory end point is identified in the external carotid artery, it should be corrected with the same sense of concern as a similar finding in the internal carotid artery. 