In 50 patients with internal carotid artery occlusion and 40 patients with internal carotid artery stenosis hemispheric and regional cerebral blood flow (rCBF) was measured by the133Xenon clearance method, rCBF was evaluated according to the presence of ischemic or hyperemic foci and to the level of regional perfusion alteration. The degree of collateral circula tion and diameter of stenosis were determined from the angiograms. The clinical symptoms were expressed in a score of motor deficiencies. In internal carotid artery occlusion no correla tion of motor deficiency index with angiographically demonstrable collateral circulation was detectable, but there was good correlation with hemispheric CBF and an excellent correlation (P < 0.001) with the rCBF parameters. In internal carotid artery stenosis no correlation existed between motor deficiency and the degree of stenosis or hemispheric CBF, but good correlation was observed between motor deficiency and the rCBF parameters (P < 0.01).According to these findings, angiographical demonstration of collateral circulation and of stenosis does not provide as accurate information on impending or already developed tissue in farction as does rCBF measurement. Presence of hyperemic foci and lack of ischemic foci were related to lesser motor disturbances. Borderlines of 30 ml/100 gm per minute for threatening tissue ischemia and of 25 ml/100 gm per minute for present tissue ischemia are established. Some points favor the thromboembolic theory of infarction in internal carotid artery stenosis.