It is widely accepted that transient cerebral ischemia and permanent stroke frequently are caused by platelet and thrombotic cerebral emboli that originate from lesions at the carotid bifurcation. Microembolization from ulcerated atheroma during carotid dissection also offers a logical explanation for the incidence of intraoperative neurologic deficits during carotid endarterectomy. The risk of intraoperative embolization is obvious when ulcers are macroscopic; but focal cerebral ischemia associated with atheroma that appear smooth and nonulcerated usually has been attributed to decreased regional cerebral blood flow. Several endarterectomy specimens were submitted for scanning electron microscopic evaluation using X20 through X4,000 malignification. Results indicate that carotid atheroma may contain superficial ulcerations and thrombi that are not appreciated by direct inspection or conventional microscopic techniques. Electron microscopic ulcerations and intraluminal thrombi may be responsible for embolic transient ischemic attacks, spontaneous strokes, and intraoperative neurologic deficits in patients in whom gross ulcerations are absent.