We used decision analysis to approach the dilemma "surgery or not?" in patients with incidental intracranial saccular aneurysms. To decide if a patient should undergo surgery, the following factors are of primary importance: age and general health of the patient, chance of rupture of the aneurysm, surgical mortality and morbidity, and patient preference for immediate and long-term risks. In a fictitious patient (a woman of 45 with migraine, otherwise healthy), surgery was the option with the greatest expected utility. Sensitivity analysis showed the influence of different factors. Other patients are also discussed, and "break-even" points for age versus surgical results are given.