OBJECTIVE: The purpose of our study is to examine the clinical significance of vertebrobasilar artery(VBA) fenestration and duplication. In addition, we review its incidence and pathogenesis. METHODS: Cerebral angiography was performed in 803 patients and magnetic resonance angiography(MRA) in 880; the patients had or were suspected to have cerebrovascular disease. We retrospectively reviewed angiography and MRA. RESULTS: Fifteen patients (eight men, seven women, 3 to 77 years of age, median age = 58 years) had a VBA fenestration and duplication. Seven (7/803 = 0.87%) of the patients undergoing cerebral angiography revealed fenestrations and one duplication of VBA. Ten patients (10/880 = 1.14%) among 880 patients that underwent MRA demonstrated fenestration of basilar artery(BA). Two of 66 patients that underwent both conventional cerebral angiography and cranial MRA showed a fenestration of BA. Twelve fenestrations were located in the proximal portion of the BA and one was in the mid portion of the BA. One vertebral artery(VA) fenestration was located in the intracranial portion of the right VA, and one VA duplication was at the level of C1-2 in the left VA. CONCLUSION: In addition to medial defects, flow phenomena at the proximal end of fenestrations, where hemodynamic stress and increased turbulence are present, may contribute to aneurysm formation. And arterial fenestration is a predisposing factor in vascular injury and cerebral ischemia.