OBJECTIVE AND IMPORTANCE: Aneurysms of the paraclinoid segment arising from the anterolateral wall of the proximal internal carotid artery (ICA) are usually located in the intradural space, and the proximal neck of the aneurysm is closely adjacent to the dural ring. Although most of these aneurysms can be identified by cerebral angiography, the top of the aneurysm can be flattened because of its relationship to the undersurface of the clinoid process. We report a rare case of a ruptured ICA anterolateral wall aneurysm that was not identified by initial angiography but was clearly observed at the time of vasospasm. CLINICAL PRESENTATION: A 59-year-old female patient was transferred to our hospital because of sudden loss of consciousness. Initial angiography failed to disclose any aneurysm except for one projecting to the right at the C3-C4 portion of the left ICA. A left frontotemporal craniotomy with drilling of the anterior clinoid process was performed 1 day after the onset of subarachnoid hemorrhage. Within the operative field, however, no hemorrhage was detected near the C3-C4 portion of the left ICA. Subsequent angiography performed during vasospasm clearly demonstrated a ruptured aneurysm in the anterior paraclinoid segment of the right ICA. INTERVENTION: After drilling the anterior clinoid process and cutting the dural ring, we successfully clipped the aneurysm. The aneurysm was saccular, with a broad neck that extended below the dural ring. Intraoperative inspection demonstrated that the dome of the aneurysm was in a subclinoid location, suggesting that most of the aneurysm might have been flattened by the dural ring at the time of initial angiography but before the onset of vasospasm. CONCLUSION: The presented case suggests that this type of aneurysm may be missed by routine angiography performed before and after vasospasm. If initial angiography fails to reveal the origin of the subarachnoid hemorrhage but computed tomography still discloses hemorrhage from an ICA aneurysm, repeated angiography should be considered within 14 days after the onset of subarachnoid hemorrhage to confirm whether the aneurysm has arisen from the anterior clinoid segment of the ICA.