Two cases of aneurysm occurring on a fenestrated basilar artery are reported. One patient had a saccular aneurysm in the typical location at the vertebrobasilar junction at the proximal end of the fenestrated basilar segment. The aneurysm sac projected dorsally into the brain stem. The other patient had a blister-like aneurysm distal to a fenestration of the terminal basilar artery. The development of a distal aneurysm on a fenestrated basilar artery is predictable based on the reported morphology of the vessel wall in this vascular anomaly.