A group of 274 patients with a single intracranial aneurysm was randomly allocated to intracranial surgery. Forty patients had no surgical procedure. Only six had a vertebral-basilar aneurysm. The remaining 228 patients had an aneurysm on the anterior portion of the circle of Willis. Mortality during the six and one-half-year interval was 36.8%. When the operation was performed within 14 days following the last bleed, mortality was 44.5%; for those operated upon after the 14-day interval mortality was 23.2%. Postoperative complications occurred in 47.8%.