We present a rare case of agenesis of the left internal carotid artery in a 43-year-old woman, associated with an aneurysm of the anterior communicating artery and presenting with subarachnoid hemorrhage. The left internal carotid artery was not visualized on the left carotid angiogram. The left middle cerebral artery was perfused from the basilar artery via the dilated posterior communicating artery on vertebral angiogram. Absence of the left carotid canal was proven on temporal bone computed tomography. Absence of the left internal carotid artery was verified at operation. Absence of internal carotid artery is discussed in relation to aneurysm formation.