This 27-year-old female suffered from subarachnoid hemorrhage. Angiography revealed a ruptured aneurysm of the anterior communicating artery. Unilateral absence of the right internal carotid artery was also demonstrated angiographically. Both the right anterior and middle cerebral arteries were perfused from the left carotid artery via the anterior communicating artery. Absence of the right carotid canal was proved on bone CT. Neuroradiological difficulties in differential diagnosis between agenesis and aplasia of the internal carotid artery were discussed. Subsequently, a diagnosis of adult type unilateral agenesis of the internal carotid artery with collateral circulation was made. The mechanism of association of cerebral aneurysm in patients with agenesis of the internal carotid artery was also discussed. From our experience, abnormal vascular anatomy should be carefully studied prior to direct surgery for a ruptured aneurysm in patients with unilateral agenesis of the internal carotid artery.