BACKGROUND: Anterior clinoidectomy is an established procedure used to decompress the optic nerve, mobilize the internal carotid artery (ICA), or enlarge the retrocarotid space. However, its use carries the risk of optic nerve injury. In certain surgeries, such as those for internal carotid aneurysms, propose modification to the anterior clinoidectomy for enlarging the retrocarotid space, especially in operations for ICA aneurysms. METHODS: After the anterior clinoid process (ACP) is sufficiently exposed, the internal cancellous bone or pneumatization can be removed through a small window created at its lateral edge to reveal the compact bone of the optic canal. Since the compact bone of the inferior surface facing the ICA is absent or very thin, the ACP can be removed by drilling through the anchoring compact bone with the optic canal in direct sight. RESULTS: In 10 consecutive internal carotid aneurysm cases, the ACP was successfully removed without opening of the optic canal to enlarge the retrocarotid space. CONCLUSIONS: Anterior clinoidectomy can be performed to enlarge the retrocarotid space without opening the optic canal from outside the dura.