We present two cases of persistent primitive trigeminal artery (PPTA) associated with a giant aneurysm originating at the opening of the PPTA on the internal carotid artery (ICA). In one case, opening of the PPTA occurred 4 months after balloon occlusion of the ICA, when a giant aneurysm located at the cavernous segment of the ICA was being treated. The PPTA was occluded successfully using a Guglielmi detachable coil (GDC). A vertebral artery approach was taken. This is the first report of the opening of a PPTA associated with contrast filling of the aneurysm after balloon occlusion of the parent artery. The possibility of contrast filling of the aneurysm via potential PPTA should therefore be considered in the event of an ICA aneurysm with parent artery occlusion.