Fusiform basilar aneurysm is a rare condition with elevated mortality within a few days if untreated. On the basis of clinical course, the fusiform aneurysm can be distinguished in an acute type, such as dissecting aneurysm, which usually causes subarachnoid hemorrhage or cerebral ischemia and in a chronic type with a relatively slow growth, which may evolve into a giant aneurysm leading to serious complications. We report a case of an 80-year-old man with a surgically untreated fusiform aneurysm that evolved into a giant aneurysm of the basilar artery within 4 years. The patient presented recurrent ischemic events involving the posterior circulation without aneurysmal rupture or bleeding.