The importance of early detection by various radiological techniques of asymptomatic, unruptured aneurysms as a means of preventing subarachnoid hemorrhage (SAH) is discussed in this report. Four hundred volunteers underwent clinical and radiological evaluations between March, 1988, and September, 1992. Studies included a neurological examination as well as digital subtraction cerebral angiography via a femoral arterial catheter, computerized tomography, T1- and T2-weighted magnetic resonance (MR) imaging of the whole brain, and MR angiography. The evaluation revealed 27 asymptomatic, unruptured intracranial aneurysms in 26 volunteers, for an incidence of 6.5%. The subjects ranged in age from 39 to 71 years, with an average of 55 years. The aneurysms were located on the internal carotid artery in 13 cases (48%), the anterior communicating artery in six (22%), the middle cerebral artery in six (22%), and the basilar artery in two (7%). Aneurysms ranged in size from 5 mm or less in 16 cases, 6 to 10 mm in nine, and 11 to 15 mm in one; one aneurysm was more than 15 mm, with a maximum diameter of 2 cm. Volunteers with a family history of SAH within the second degree of consanguinity showed a higher incidence of aneurysms (17.9%). Aneurysm clipping was performed on 20 of the 26 cases with no significant morbidity or mortality. These findings support the contention that aggressive early detection of unruptured aneurysms may improve the outcome in patients harboring cerebral aneurysms by preventing the devastating effects of SAH.