Fusiform aneurysms of the posterior circulation constitute a relatively uncommon subgroup of aneurysms. We present a patient with fusiform basilar artery aneurysm accompanied by ectasia and elongation of both supraclinoid parts of the internal carotid arteries, middle cerebral arteries and vertebral arteries. The aneurysm had thrombus formation along its length which progressed to occlude the lumen the day after the patient's admission. Mass effect, progressive infarcts at the territories of posterior inferior cerebellar (PICA) and superior cerebellar (SCA) arteries and brainstem perforators along with subarachnoid and subdural hemorrhage were present.