Saccular aneurysm of the cerebral arteries occur in 1 to 5% of the population; the major risk of this disease is aneurysm rupture causing subarachnoid hemorrhage associated with a mortality rate of 50 to 60%. Two methods exist to treat cerebral aneurysms: neurosurgical clipping at the base of the aneurysm and endovascular introduction of a platin coils. Both methods have advantages and disadvantages. With the knowledge of the flow in an aneurysm it is easier to choose the appropriate method. In this study we simulated the flow in four different aneurysms. The shape of the aneurysms are gained from computertomographic data. The simulated flow in four aneurysms shows the great variability of possible flow patterns.