Ninety percent of cerebral aneurysms were diagnosed after their first rupture with consecutive subarachnoid haemorrhage. In order to detect the aneurysms before rupture investigation by transcranial Doppler sonography was used on the basis of intra-operative evaluation of the normal aneurysmal flow pattern. In 34 patients the directly registered aneurysmal flow pattern was characterized by mean velocities slower than in parent arteries, by low diastolic flow velocities and by additional peaks superimposed on the normal pulse curve. The transcranial investigation with a large sample volume which includes aneurysm and parent artery was not able to distinguish this aneurysmal flow pattern from the normal one. Only in large to giant aneurysms a pathological flow pattern could be recorded transcranially.