A 69-year-old woman suffered subarachnoid hemorrhage due to rupture of an aneurysm at the tip of the basilar artery. The aneurysm was treated by endovascular treatment using Guglielmi detachable coils but the aneurysm dome could not be completely packed. Serial angiography at 10 and 18 months after embolization showed progressive regrowth of the aneurysm with loosening and unraveling of the packed coils. The patient died accidentally after head injury and autopsy was performed. Examination of the aneurysm showed further regrowth and "relative coil compaction" of the coil mass. Histological examination of the resected aneurysm showed no endothelialized membrane in the orifice and only minimal organized thrombus in the body of the aneurysm.