The flow pattern and the paths of fluid particles in a saccular aneurysm located at the bifurcation of an intracranial arterial segment are investigated with a numerical method. A normal physiological flow pattern was assumed as input to the studied segment. The theoretical study is carried out for two different Reynolds numbers and two different geometries of the aneurysm. The governing equations for incompressible Newtonian fluid flow are solved using the finite element method. The results show the disturbed blood flow in the pathologically altered bifurcation and the flow activity in the aneurysms. It is particularly important that blood particles can circulate in a whirl within the aneurysm for a time which seems long enough to permit the generation of cell aggregates or/and blood clots.