Flow characteristics are examined in two lateral model aneurysms by means of numerical simulation. The study concentrates on basic flow and stress patterns in a rigid wall and in a distensible wall aneurysm. The numerical solution of the governing Navier-Stokes equations describing incompressible, pulsatile, three-dimensional non-Newtonian flow is accomplished with the use of a finite element method together with a pressure correction technique. The inflow into the aneurysm is seen to arise from the downstream lip of the orifice and to be directed backward to the center. Backflow to the parent vessel takes place along the walls of the aneurysm. The intra-aneurysmal flow is found to be low compared with the flow velocity in the parent vessel, and even stagnation of flow occurs in the dome of the aneurysm. With a distensible wall, the basic flow characteristics are changed during systolic flow. The intra-aneurysmal secondary flow increases significantly. The increase and decrease of the flow velocity at the downstream lip reflect the expansion and contraction of the aneurysm wall where the maximal wall displacement during systolic acceleration is about 6% of the aneurysm diameter.