In order to investigate the developmental mechanism of cerebral aneurysms, the in vivo flow pattern around human cervical carotid bifurcations was studied by flow visualization using digital subtraction angiography with an isotonic contrast medium. The blood stream containing the medium impinged on the apex, then proceeded along the walls of the branches. After opacification of the whole lumen around the apex, most of the medium was carried away, while some remained for a few seconds at the carotid sinus. In the internal carotid artery, the blood struck the wall at an oblique angle near the tops of the arterial curvatures. In cases with atheromatous plaque or kinking of the branch, the blood passed through the stenosed segment and moved upstream, indicating turbulence. The study suggests that haemodynamic forces around the apex consist mainly of impingement on the apex and shear stress to the wall at and around the apex. In branches, high shear stress seems to exist. It might be possible that high shear stress causes degenerative changes in the endothelial layer, initiating the formation of saccular and fusiform cerebral aneurysms.