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 = 114%] 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 $C_{1-2}$ 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.