Aneurysmal subarachnoid hemorrhage (SAH) carries a grim prognosis, with high mortality and morbidity rates. The mortality rate in the first 30 days postrupture is estimated to be in the range of 40 to 50%, and almost half of the survivors will be left with a neurological deficit. Unlike patients with aneurysmal SAH, those with unruptured intracranial aneurysms usually experience no neurological deficit, and their treatment is prophylactic, aiming to reduce the risk of future bleeding and its consequences. The risk of rupture therefore assumes special importance when making decisions regarding which patient or aneurysm to treat. In previous reports the risk of bleeding for unruptured aneurysms has been stated as approximately 2% per year. The retrospective part of the International Study of Unruptured Intracranial Aneurysms (ISUIA) reported very low annual bleeding rates (0.05-1%) and high surgical morbidity and mortality rates (8-18%), prompting discussion in which the benefits of prophylactic treatment in the majority of these lesions were questioned. Prospective data from the second part of the ISUIA recently included rupture rates ranging from 0 to 10% per year. The aim of this paper was to review the evidence that is currently available for neurosurgeons to use when making decisions regarding patients who would benefit from treatment of an unruptured intracranial aneurysm.