In almost one in six patients with spontaneous subarachnoid hemorrhage(SAH) no lesion responsible for the bleed will be found by cerebral angiography. Current management strategies include repeat cerebral angiography after a period of 1-8 weeks during which a number of these patients will rebleed with considerable morbidity and even mortality. We report the case of a 51-year-old female patient with spontaneous SAH. Brain computed tomography (CT) demonstrated localized focal hematoma in the basal frontal interhemispheric fissure suggesting the presence of an anterior communicating artery(ACoA) aneurysm. Cerebral angiography was incomplete because of severe arther-osclerosis of left internal carotid artery and findings were negative. Cerebral three-dimensional computed tomographic angiography(3-D CTA) performed the next day showed a small aneurym arising from the ACoA. This was confirmed at surgery. It is suggested that 3-D CTA, guided by the CT findings, probably is useful in the diagnosis of patients with acute SAH and has a place in the management of patient with SAH of "unknown etiology" before repeat catheter angiography is undertaken.