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We have previously reported that the topographic variability of the territories of the anterior, middle, and posterior cerebral arteries is much larger than is generally considered in the literature. In the current study, we quantitatively investigated the variability of the territorial distribution of the major cerebral arteries and analyzed its relation to the variability of the circle of Willis.In 23 human brains, the volumes of the major cerebral territories were calculated and the diameters of the contributing arteries were measured and standardized for size.The variability of the standardized territorial volumes proved to be considerably large and strongly correlated with the variability of the diameters of the arteries emanating from the circle of Willis. Furthermore, the relative vascular densities in the gray and white matter were determined. Taking these densities into account, an estimation of the relative peripheral resistance of the contributing arteries was made.We hypothesize that the morphological variability of the cerebral vascular system is related to the peripheral resistance of the major cerebral arteries and, consequently, to flow patterns, both of which are hemodynamic factors. We suggest that hemodynamic factors predominantly determine the form and size of the cerebral vascular system.