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Twenty-eight internal carotid artery stenoses in 26 patients were studied. The smallest radius of the lumen at the stenosis was determined from the angiogram. The resistance of the stenosis was calculated from the pressure difference and blood flow measured at the time of vessel reconstruction. Stenoses with small radii were found to have high resistance and vice versa. In stenoses with radii above 50% of the normal vessel radius, the vessel reconstruction rarely produced any blood flow increase. In stenoses with radii below 50% of normal, there was often a blood flow increase, amounting to 40% on the average. The relationship between radius and resistance did not follow the law of Poiseuille. The resistance was found to be approximately ten times higher than predicted when calculated from the radius according to this law. The resistance was dependent upon a power of the radius between the third and the fifth.