A 38-year-old housewife presented with a ruptured aneurysm associated with unilateral internal carotid artery agenesis. She had been in good health until May 31, 1994, when she was admitted to our facility immediately after sudden onset of headache and nausea. She was alert and exhibited no focal neurological deficit on admission. Cerebral angiography demonstrated an aneurysm arising from the junction of the horizontal segment of the right posterior cerebral artery and posterior communicating artery. The right internal carotid artery was totally absent. High resolution computed tomography demonstrated absence of the right carotid canal in the skull base. Neck clipping of the aneurysm was carried out through the right pterional approach on June 2. She returned home 52 days after the surgery with mild paresis of the left upper extremity and has since resumed household activities. Early surgery may be recommended in a patient with a ruptured aneurysm associated with agenesis of the internal carotid artery to prevent catastrophic rebleeding, if the initial insult is mild and subsequent vasospasm is unlikely to occur.