In view of the widely divergent opinions regarding the necessity for operation in cases of asymptomatic and multiple cerebral aneurysms, an attempt is made to derive the true lifetime risk of rupture for a cerebral aneurysm in the asymptomatic patient. Prevalence data from autopsy and angiographic studies are utilized to derive a lifetime incidence for the rupture of such asymptomatic aneurysms. A decremental life table analysis method is used. The case of multiple asymptomatic aneurysms in a patient with another symptomatic aneurysm must be considered separately. Implications of these calculations are drawn regarding operation for unruptured or incidental aneurysms.