Of 59 cases of vertebrobasilar junction aneurysms diagnosed and treated from January 1977 to April 1986, 21 (35.5%) saccular aneurysms arose in a fenestration of the proximal basilar artery. Defects of the media at the junctures of the fenestrated segments, as well as the possible presence of turbulent flow at the vertebrobasilar junction, may explain the high incidence of vertebrobasilar aneurysms associated with proximal basilar artery fenestration.