OBJECTIVE: We examined the incidence of fenestration in the proximal segment of the anterior cerebral artery(ACA) and reviewed its clinical significance. METHODS: Cerebral angiography was performed in 843 patients and magnetic resonance angiography(MRA) in 1,787; all patients had, or were suspected to have, cerebrovascular disease. We subsequently reviewed the angiography and MRA data. RESULTS: Fourteen patients (six men and eight women, 35~81 years of age, median age = 67 years) had proximal ACA fenestrations. Three of the 843 patients (0.36%) undergoing cerebral angiography had fenestrations of proximal ACA as did eleven of the 1,787 MRA patients (0.62%). Seven fenestrations were located on the right side and seven fenestrations were located on the left side. The fenestrated ACA in one patient was associated with a saccular aneurysm at its proximal end. None of the 120 patients who underwent both conventional cerebral angiography and cranial MRA showed fenestration of proximal ACA. CONCLUSION: Although fenestration of ACA has little clinical significance, knowledge and recognition of this condition is important in the interpretation of cerebral images, especially during neurosurgical procedures.