We reviewed all patients over a 3-year period with documented aneurysms of the posterior cerebral circulation, and re-examined the angiograms of 103 patients. There were 9 cases of fenestration of the basilar artery, all associated with aneurysms of the basilar artery. In 6, the aneurysm arose at the site of the fenestration. We believe that the presence of a fenestration predisposes to aneurysm formation, due to the combination of the resulting alteration in flow characteristics and the presence of a defect in the media of the vessel.