Abstract A stenosing atherosclerotic plaque of the internal carotid artery acts as a source of emboli and may impede flow. The contribution made by a stenosed carotid artery to cerebral perfusion is unclear. This study examined blood flow in the internal carotid artery and a comparison was made with the effect of clamping on flow velocity in the middle cerebral artery. Some 50 patients were assessed before carotid endarterectomy by preoperative duplex scanning, triplanar arteriography and transcranial Doppler ultrasonography. At operation, maximal flow in the internal carotid artery was measured using a Doppler flowmeter and flow velocity was monitored by transcranial Doppler. No correlation was found between the degree of stenosis and resting flow velocity. Mean flow in the internal carotid artery in patients with >90% stenosis was significantly lower than those with lesser stenoses (P < 0.01). However, a mean minimal blood flow of 141 ml/min passed through the >90% stenoses. The effect of internal carotid artery clamping on flow velocity was similar irrespective of the degree of stenosis. In patients with >90% stenosis, the internal carotid artery continues to make an important contribution to cerebral perfusion.