Among 6,638 cases reported to the Cooperative Study of Intracranial Aneurysms and Subarachnoid Hemorrhage were 477 cases in which the cause of hemorrhage could not be determined after carotid and vertebral angiography. These patients were followed up for up to 24 years after hemorrhage. Twenty patients were subsequently found to have an aneurysm or arteriovenous malformation missed by the first angiographic survey. After six-month survival, the rate of recurrent hemorrhage was a maximum 0.86% per year. Survival was significantly better than that of patients with verified ruptured aneurysms managed conservatively in this cooperative study. For normotensive patients who survived the first six months, the life expectancy for the next 20 years equaled that of an age- and sex-matched US population. Hypertensive patients had a higher mortality than normotensive patients.