Monitoring of cerebral perfusion by near-infrared spectroscopy estimates regional cerebral oxygen saturation (rSO2). We present a case in which, before clamping the left carotid artery during an endarterectomy, the right and left rSO2 measurements were 72% and 74%, respectively. Within 15 seconds of clamping the external carotid artery, the left rSO2 decreased by 8%, yielding right and left rSO2 measurements of 70% and 66%, respectively. No electroencephalogram changes ensued. The internal carotid artery was clamped 1 minute later, whereas the external carotid remained clamped. No electroencephalogram changes were observed. The rSO2 measurements demonstrate that the value of this cerebral oximetry is not determined solely from internal carotid blood flow and can be significantly affected by the external carotid.