Patients with a giant intracranial aneurysm usually present as having an intracranial space-taking lesion, but no subarachnoid hemorrhage. The authors describe a 61-year-old man with a completely thrombosed giant aneurysm at the pericallosal artery. Computed tomography scan showed a large, round, mottled, high-density area in the paramedian frontal region that had ringlike enhancement with contrast media. On cerebral angiography it was demonstrated to be an avascular mass lesion. These findings led to an initial diagnosis of falx meningioma. However, final diagnosis at operation was completely thrombosed giant aneurysm. The literature is reviewed and the pathogenesis of intracranial thrombosed giant aneurysm, in regard to its growth and thrombus formation, is discussed.