Reperfusion brain edema occurs infrequently after carotid endarterectomy and has been reported only ipsilateral to the side of surgery. We report a 51-year-old woman who presented with transient right arm weakness followed by left hemiparesis. Angiography showed occlusion of the left internal carotid artery, 90% stenosis of the right internal carotid artery, filling of the left anterior cerebral artery from the right carotid circulation only, and filling of the left middle cerebral artery branches by pial collaterals from the left anterior cerebral and posterior cerebral arteries. The patient had a right carotid endarterectomy and 1 day postoperatively developed a severe headache but had no new focal neurologic findings. Computed tomography showed effacement of sulci and scattered areas of high attenuation in the left parietal lobe, consistent with cerebral edema and petechial hemorrhage or vascular engorgement. Magnetic resonance imaging 2 days later did not show a lesion in the left hemisphere, suggesting resolution of the edema. This report suggests that reperfusion brain edema can occur contralateral to carotid endarterectomy.