The authors summarize the findings of previous studies relating to the natural history of aneurysms and arteriovenous malformations (AVM's). Ruptured aneurysms have their highest rate of rebleeding on Day 1, and at least 50% will rebleed during the 6 months after the first hemorrhage. Thereafter, the rate drops to at least 3% a year. This is the same rate as seen in anterior and posterior communicating artery aneurysms treated by anterior cerebral artery clipping and carotid ligation; these operations provide immediate protection but do not result in long-term diminution of the risk of rebleeding. Patients with unruptured incidental and unruptured multiple aneurysms rebleed at a rate of 1% per year, as do patients with subarachnoid hemorrhage of unknown etiology. The risk of rebleeding for AVM's is 3% a year.