OBJECTIVE: The expansion of endovascular procedures for obliteration of cerebral aneurysms highlights one of the drawbacks of clip ligation through the transcranial route, namely brain retraction or brain transgression. Sporadic case reports have emerged over the past 10 years describing endonasal endoscopic clip ligation of cerebral aneurysms. The authors present a detailed anatomical study to evaluate the feasibility of an endoscopic endonasal approach for application of aneurysm clips. METHODS: Nine human cadaveric head specimens were used to evaluate operative exposures for clip ligation of aneurysms in feasible anterior and posterior circulation locations. Measurements of trajectories were completed using a navigation system to calculate skull base craniectomy size, corridor space, and the surgeon's ability to gain proximal and distal control of parent vessels. RESULTS: In each of the 9 cadaveric heads, excellent exposure of the target vessels was achieved. The transplanum, transtuberculum, and transcavernous approaches were used to explore the feasibility of anterior circulation access. Application of aneurysm clips was readily possible to the ophthalmic artery, A1 and A2 segments of the anterior cerebral artery, anterior communicating artery complex, and the paraclinoid and paraclival internal carotid artery. The transclival approach was explored, and clips were successfully deployed along the proximal branches of the vertebrobasilar system and basilar trunk and bifurcation. The median sizes of skull base craniectomy necessary for exposure of the anterior communicating artery complex and basilar tip were 3.24 cm(2) and 4.62 cm(2), respectively. The mean angles of surgical corridors to the anterior communicating artery complex and basilar tip were 11.4° and 14°, respectively. Although clip placement was feasible on the basilar artery and its branches, the associated perforating arteries were difficult to visualize, posing unexpected difficulty for safe clip application, with the exception of ventrolateral-pointing aneurysms. CONCLUSIONS: The authors characterize the feasibility of endonasal endoscopic clip ligation of aneurysms involving the paraclinoid, anterior communicating, and basilar arteries and proximal control of the paraclival internal carotid artery. The endoscopic approach should be initially considered for nonruptured aneurysms involving the paraclinoid and anterior communicating arteries, as well as ventrolateral basilar trunk aneurysms. Clinical experience will be mandatory to determine the applicability of this approach in practice.