The results of operation have been analysed in 182 patients with 219 intracranial arterial aneurysms. One hundred and sixty of the aneurysms had ruptured; for 150 "typical" supratentorial aneurysms, the operative case fatality rate was 10%, and a satisfactory outcome was obtained in 85%. There were no deaths and little morbidity in 108 operations on grade 0 or 1 patients. Among a number of factors influencing outcome, the most obvious were age, hypertension, cerebral arterial spasm, and the clinical condition of the patient at the time of operation. The timing of surgery had no effect on results.