The late natural history of ruptured intracranial aneurysms was studied in 568 cases reported to the Cooperative Study of Intracranial Aneurysms and Subarachnoid Hemorrhage from 1958 to 1965. The patients had been selected for conservative management of their aneurysms at the time of diagnosis. A follow-up search in 1981 and 1982 revealed 378 known deaths; 40% had occurred within six months of hemorrhage. During the next two decades, the patients' survival probabilities were significantly worse than those of a matched US population. Multiple aneurysms did not differ prognostically from single aneurysms, but posterior circle aneurysms carried a better prognosis after ten-year survival. The rate of probable recurrent bleeding after six months was 2.2% per year for the first 9 1/2 years and 0.86% per year for the second decade. Reported rebleeding episodes were fatal in 78%.