A theoretical evaluation of the lifetime probabilities of different outcomes in patients with unruptured intracranial aneurysms (UA) has been made using a life table method. The calculations were performed for aneurysm presenting ages from 20 to 70 years of age for men and women assuming an annual risk of aneurysm rupture of 1%, 2% and 3% and a rate of mortality after rupture of 50%. At 10, 20, 30, 40, 50 and 60 years after the diagnosis of an UA the probability of survival without bleeding is reduced below the expected probability of survival according to the life tables by the following percentages (assuming an annual risk of bleeding of 2%): 19%, 34%, 46%, 56%, 64% and 72%, respectively. A survey of the lifetime probabilities of four different outcomes for patients with an UA indicates a substantial reduction in life expectancy after the diagnosis of an UA. In most ages the surgical risks are more than balanced by the risks associated with an untreated unruptured aneurysm.