OBJECTIVE To investigate whether the branching geometry determines an underlying increase of shear stress (SS) on the vessel wall in cerebral arteries of patients with aneurysms located distally to the circle of Willis. Increased SS is regarded as a major factor in the etiology of intracranial saccular aneurysm. Aneurysms occur commonly in the Willisian arteries, where the role of hemodynamic forces are evident, but they occur also in more peripheral arteries. METHODS The ratio between SS in the branches and SS in the parent vessel at bifurcations was estimated using exponential relations of vessel caliber. The absolute difference of SS branch ratios in every bifurcation represents the SS gradient at the apex. Cerebral angiograms of 10 patients with aneurysm of the distal anterior cerebral artery were analyzed and compared with normal values from an earlier study. RESULTS The branching geometry determines a relatively small but significant increase of SS in branches and of SS gradients at bifurcation apices in cerebral arteries of patients with aneurysm. CONCLUSIONS The results may reflect increased cerebral vessel tone after subarachnoidal hemorrhage, or alternatively an underlying derangement of the regulation of cerebral arterial caliber and SS in these patients.