Fenestration of basilar artery is an uncommon vascular anomaly that is usually an incidental product on autopsy or angiography. None of the cases in the literature had clinical symptoms associated with this anomaly except for subarachnoid hemorrhage when accompanied with saccular aneurysm. We report a rare case of the basilar artery fenestration associated with clinical symptoms without any aneurysm. A 71-years-old male, who had been treated for labile hypertension and had had recurrent attacks of vertigo, nausea, sometimes diplopia or unsteady gait, for 5 years, was referred to our hospital on Sept. 13, 1985. One day prior to admission, he suddenly felt diplopia and vertigo and unsteady gait. His family noticed he was dysarthric. On admission, he was alert and normotensive. He complained of dysesthesia on the right half of the perioral region and his right fingers. A neurological examination showed a mild weakness and hyperactive deep tendon reflexes on his right leg. His motor coordination was almost normal, but he was unsteady when he stood on one foot with his eyes closed. Laboratory examinations were normal except for an elevated serum uric acid level. A chest x-ray film showed a sclerotic change of aorta and mild cardiomegaly. Left ventricular hypertrophy was observed on his ECG. His CT scans showed multiple lacunae and mild brain atrophy. On cerebral angiography, his basilar artery (BA) had a fenestration almost in its total length that divided the BA, like a duplication, into two components with a smaller diameter than normal.(ABSTRACT TRUNCATED AT 250 WORDS)