During 1964-9, 178 patients with subarachnoid haemorrhage from a single intracranial arterial aneurysm were allocated at random to receive operative or conservative treatment at an average of seven weeks after bleeding. During the follow-up fatal rebleeding episodes occurred in six of the 86 patients treated surgically and 16 of the 92 treated conservatively. This difference was significant. Fatal rebleeding occurred on average 40 months after the first episode. Deaths from all causes occurred in 17 of the 86 patients treated surgically and 22 of the 92 treated conservatively. Life-table analysis of the chances of surviving 1, 5, and 11 years gave probabilities of 95 and 91%, 87 and 86%, and 76 and 75% in the two treatment groups respectively. Of the 139 patients alive after a mean follow-up of nine years, 130 (94%) were fully independent in their daily lives, and only 43 (31%) were unable to work. The method of treatment did not affect the quality of survival.The results show that fatal rebleeding may occur even many years after the first episode. Nevertheless, if the patient is in good condition seven weeks after a haemorrhage from a single intracranial arterial aneurysm the outcome is good irrespective of whether operation is performed at this late stage.