1. In the first 6 months, at least 50% of the patients with untreated anterior and posterior communicating artery aneurysms will rebleed. 2. Thereafter, the rate of rebleeding is not less than 3%/year with two-thirds of these late hemorrhages resulting in a mortality. 3. Carotid ligation for posterior communicating aneurysms protects against a high rate of rebleeding for 6 months, but thereafter the rate is not different from the course of untreated aneurysms. 4. On the basis of these first three points and a review of the literature, a theory is proposed that incidental and symptomatic unruptured aneurysms, multiple unruptured aneurysms, and subarachnoid hemorrhage where no lesion is demonstrable, have a natural history similar to that of the untreated aneurysm that survives at 6 months. Namely, we would predict a yearly rebleed rate of not less than 3%/year.