BACKGROUND Proximal arterial occlusion, with or without extracranial-to-intracranial (EC-IC) bypass, is frequently used as treatment for giant intracranial aneurysms that are unclippable. The authors report on a patient who had obliteration of a giant unruptured aneurysm of the right internal carotid terminus after undergoing an EC-IC bypass without proximal arterial ligation. METHODS This 71-year-old woman presented with repeated right cerebral ischemia caused by a giant saccular aneurysm of the right internal carotid terminus. Direct surgical clipping of the aneurysm was not recommended because of the patient's age and because of the morphology of the aneurysm. She could not tolerate occlusion of the right internal carotid artery (ICA) and, therefore, first underwent an EC-IC bypass. Four weeks later, she returned to undergo a balloon occlusion of the right ICA proximal to the aneurysm. RESULTS The right distal ICA and aneurysm were found to be spontaneously thrombosed. At 2-year follow-up, the aneurysm was shown to be completely obliterated on the magnetic resonance imaging scans. CONCLUSIONS The authors conclude that hemodynamic changes in the blood flow of the parent artery after EC-IC bypass caused this occurrence.