UP TO 6% of cerebral aneurysms may be familial. Because the pattern of inheritance and the prevalence of aneurysms within families are unknown, the management of family members at risk of harboring a cerebral aneurysm is currently empirical. We established the prevalence of aneurysms in the second generation of individuals with familial cerebral aneurysms and determined the possible benefit of angiographic screening and elective surgery of such individuals by using a simple decision analysis model. Four consecutive families were identified in whom the mother and a child had a ruptured cerebral aneurysm. A total of 19 siblings at risk in the second generation were identified. Fifteen underwent elective cerebral angiography: one had a cerebral aneurysm and two had an infundibulum at the origin of the posterior communicating artery. Including the previously known aneurysms, the prevalence of aneurysms in the second generation was thus established at 29.4%. A decision analysis was performed with 2% as the annual risk of rupture, 72.7% as the risk of death or disability with rupture, 0.1% as the risk of angiography, and 6.5% as the risk of surgery. The benefit in years of survival free of sequelae resulting from angiographic screening and elective surgery (intervention) over natural history was computed for life expectancy corresponding to each quinquennial age group from age 15 to 100 years. Intervention equaled natural history, in terms of years of survival expected with each choice, at a life expectancy of 10.6 years, corresponding to age 76.5 years for men and 80 years for women, and produced a net gain of at least 1 year for patients whose life expectancy was 32 years or more, corresponding to age 53.5 years for women and 49 years for men. Greater benefit was achieved with increasing life expectancy (younger age). The prevalence of aneurysms in the second generation when a mother and child have an aneurysm is 29.4%. Intervention produces a benefit of at least 1 year of survival free of sequelae over natural history in such individuals if their life expectancy is 32 years or more.