The increased sensitivity of neuroimaging techniques has enabled the more frequent diagnosis of unruptured aneurysms. Because the most devastating complication of an unruptured aneurysm is subarachnoid hemorrhage, it has been considered desirable to treat these aneurysms before they rupture. However, the optimal treatment strategy for patients with unruptured aneurysms remains controversial. The management decision requires knowledge of the natural history and an accurate assessment of the risks related to various treatment options. On December 10, 1998 the New England Journal of Medicine published an article by the International Study of Unruptured Intracranial Aneurysms Investigators. The study retrospectively assessed the natural history of unruptured aneurysms in 1449 patients; in addition, treatment-related mortality and morbidity were examined in a prospective cohort of 1172 patients. The major finding was that the rate of rupture for aneurysms smaller than 10 mm in diameter in patients with no history of subarachnoid hemorrhage was 0.05% per year, and that the mortality and morbidity associated with surgery greatly exceeded the risk of rupture in such patients. This report has generated tremendous controversy in the treatment of unruptured aneurysms and has influenced our decision making. The results of this study are critically analyzed and potential flaws presented. To develop recommendations for treatment, the literature was reviewed. The conclusion of this search is that there is insufficient evidence to recommend a standard of management. Therapeutic guidelines are provided, particularly for patients with small incidental aneurysms and no history of subarachnoid hemorrhage.