This 70-year-old female was admitted to our hospital 1 day after a sudden consciousness disturbance. Computed tomography (CT) showed subarachnoid hemorrhage (SAH), and angiogram revealed an irregular-shaped aneurysm at the lateral medullary segment of the left posterior inferior cerebellar artery (PICA). The patient was treated by intra-aneurysmal embolization with Guglielmi detachable coil (GDC) with parent artery preservation. Post-operative angiogram showed obliteration of the aneurysm except for the neck remnant, but she presented with rerupture 19 days after the onset and died 3 days later. Postmortem examination revealed massive hematoma around the aneurysm, which compressed medulla oblongata from behind. Histological assessment showed the 'entry' where the aneurysmal wall lacked internal elastic lamina, providing evidence of dissecting aneurysm. The present case suggests that embolization of distal PICA aneurysm with parent artery preservation should be avoided because radiological evaluation may fail to rule out the possibility of dissection, where the aneurysmal wall is affected not only at the 'entry' but also in the adjacent region.