We evaluated prospectively the relation between packing and reopening in coiled intracranial aneurysms. Packing, defined as the ratio between the volume of inserted coils and volume of the aneurysm expressed as percentage, was calculated for 82 intracranial aneurysms treated with detachable coils. Aneurysm volume was assessed from 3D angiography. Reopening of the aneurysmal lumen at the 6-month follow-up angiography was dichotomized into present or absent. We assessed whether packing above 24% protected against reopening. Twenty-three of 82 aneurysms (28%) showed reopening. Reopening was caused by compaction in 20 aneurysms and by partial thrombosis, undetected at the time of initial treatment in three aneurysms. Three of 29 aneurysms (10%) with a packing of more than 24% showed reopening. These three aneurysms contained partially intraluminal thrombosis undetected at the time of treatment. We conclude that in coiled intracranial aneurysms packing above 24% protects against reopening by compaction in non-thrombosed aneurysms. Since intraluminal thrombosis may go undetected at the time of treatment, follow-up angiography is still warranted in aneurysms with packing densities greater than 24%.