In this article we present an 80 year old female patient with an unruptured giant aneurysm of the basilar artery presenting with posterior circulation ischemic symptoms. Angiography and CT revealed giant basilar aneurysmal dilatation with severe and wide intracranial arteriosclerosis. We described the uniqueness of this case. Giant basilar aneurysm is associated with various complications particularly brain stem infarction. It is emphasized that arteriosclerosis plays an important role in the formation of giant basilar aneurysms.