Giant intracranial aneurysm, approximately 5-7% of all intracranial aneurysm, is defined as aneurysm with a maximal sac diameter reaching or exceeding 25 mm. Its high incidence of thrombus formation causes difficulty in diagnosis through imaging studies. Recently, we have experienced a giant contralateral internal carotid artery aneurysm that was completely thrombosed and negative in imaging studies. It was only verified during surgery for clipping of an anterior communicating artery aneurysm. On initial cerebral angiography, authors had not found any other aneurysm except a small-sized anterior communicating artery aneurysm. There was no evidence of aneurysm or mass on right internal carotid artery even though in postoperative computed tomographic angiography (CTA). We emphasize the fact that another completely thrombosed aneurysm may be present as a hidden lesion during initially planned aneurysmal surgery.