A 50-year-old man and a 69-year-old woman with consciousness disturbance were diagnosed to have suffered from subarachnoid hemorrhage (SAH) involving the posterior fossa. In both cases, the initial 3D CT angiogram failed to reveal the SAH source in the vertebrobasilar system. Delayed 3D rotational angiography revealed aneurysms on unfamiliar aberrant arteries. One was a dissecting aneurysm located between the proximal part of the posterior inferior cerebellar artery and the distal part of the intracranial vertebral artery. It was trapped and resected; the patient subsequently presented with lateral medullary symptoms. The other aneurysm was between the distal posterior and the distal anterior inferior cerebellar artery. It was successfully embolized; there were no complications. We think that the aberrant aneurysm-harboring vessels encountered in these two patients were primitive arteries on or adjacent to the vertebrobasilar paramedian longitudinal axis and that they persisted past the embryologic stage. Such aneurysms arising from unfamiliar persistent arteries beside the brainstem are extremely rare but must be considered when the SAH source is not detected in the trunk of the vertebrobasilar system.