Unruptured aneurysms were diagnosed in 15 of 86 patients with cerebral aneurysms during 2 years beginning in April, 1979. One patient with severe head injury was excluded from the series. Fifteen aneurysms in the other 14 patients were first detected by computerized angiotomography. Six aneurysms were located in the middle cerebral artery, three in the upper half of the basilar artery, two in the anterior communicating artery, two in the posterior cerebral artery, and two at the internal carotid-posterior communicating artery junction (both in the same patient). Three were smaller than 5 mm, eight were between 6 and 10 mm, and four were larger than 10 mm. The noncontrast computerized tomography (CT) findings associated with 15 aneurysms were as follows: five showed defects in the basal cistern of Sylvian fissure, four were calcified or high-density masses, and in the six instances there was no evidence of an aneurysm. Unruptured aneurysms may be suggested by a well demarcated, round, isodense mass which forms a defect in the basal cistern of Sylvian fissure on a plain CT image, and are highly and homogeneously enhanced by computerized angiotomography. A carotid artery blood iodine level of 15 mg/ml is required to obtain clear images. The authors conclude that computerized angiotomography is useful in the delineation of unruptured aneurysms.