Three hundred sixty-four patients who suffered a subarachnoid hemorrhage from an aneurysm in either the posterior communicating or anterior communicating artery and who were not surgically treated have been followed for up to 21 years in order to determine the frequency of subsequent hemorrhage. For patients surviving six months: (1) rebleeding occurs on the average of 3.5% per year during the first decade; (2) the mortality associated with a late rebleed is 67%; (3) high blood pressure and female sex predispose to late rebleeding in posterior communicating artery aneurysms; (4) young patients take longer to rebleed than older patients but do so at the same rate; and (5) increase in aneurysm size as judged by routine arteriography six months after the initial hemorrhage did not correlate with rebleeding, although an increase in size had occurred in all patients who were studied angiographically at the time of their late rebleed.