The authors reviewed the computed tomographic (CT) scans of patients with subarachnoid hemorrhage whose initial angiograms were negative, to investigate the validity of CT scans in predicting the presence of an angiographically missed aneurysm in such patients. During the past 14 years, additional angiograms have been obtained for 38 of the 45 patients with subarachnoid hemorrhage whose initial angiograms disclosed no aneurysm. Aneurysms were found in 8 patients; 7 on the anterior communicating artery and 1 at the junction of the internal carotid and posterior communicating arteries. CT scans were taken within 4 days after subarachnoid hemorrhage in 31 patients. Analysis of these scans showed that the second angiogram revealed 1) an aneurysm in 21% of the patients with a thin layer of subarachnoid blood and in 63% of those with a thick layer; 2) no aneurysm in the patients without subarachnoid blood; and 3) an aneurysm of the anterior communicating artery in 70% of the patients who showed a considerable amount of blood in the basal frontal interhemispheric fissure. These results suggest that if CT scans show thin or thick subarachnoid blood, angiographic study should be repeated early in the course. If a considerable amount of blood is shown in the basal frontal interhemispheric fissure, it is highly probable that an aneurysm is hidden on the anterior communicating artery, even if the angiogram is negative for an aneurysm.