We performed light microscopic examinations on 76 aneurysms from 72 patients (multiple aneurysms in four patients). The slides were stained with HE, Elastica-van-Gieson, Goldner and Prussian blue for iron. Based on morphological findings we were able to differentiate 4 groups of aneurysms. Aneurysmal walls frequently showed complete fibrosis or intimal hyperplasia. One third of the patients had prodromal signs such as headache or neurological deficits before the first subarachnoidal haemorrhage. These aneurysmal walls also had atherosclerotic changes in most cases. Haemodynamic stress as a causal factor seemed to be very important in aneurysmal wall formation. Four patients presented with giant aneurysms with thick fibrotic walls and intimal hyperplasia. 11 aneurysms had never bled. Their walls showed likewise atherosclerotic changes, intimal hyperplasia and sometimes haemosiderin deposits indicating earlier, local, clinically silent bleeding. In aneurysms after one or multiple bleedings, granulocytes and lymphocytes and partly organized thrombi could be found displaying varying degrees of reparative processes. There is a slight positive correlation between the degree of potential reparative processes and the time intervals between bleeding and histological investigation. Therefore obviously, local reparative processes stabilizing the aneurysmal wall may take place in aneurysms even after bleeding.