Berry aneurysms are peculiarly common to the cerebral circulation of man. They are acquired lesions related to the large calibre and atherosclerosis of the cerebral arteries. The aetiology and pathogenesis are intimately concerned with haemodynamic stresses at arterial forks without scientific evidence of maldevelopmental errors. Whilst connective tissue disorders may predispose to their development, they are not essential aetiological factors. There is a need to identify the particular haemodynamic stresses and biophysical responses of the mural connective tissues in their development.