This prospective study comprises all patients treated for spontaneous non-traumatic subarachnoid haemorrhage between May 1991 and December 1995 in the county of Vestfold, Norway. A total of 76 patients was recorded, giving an incidence of 8.1 per 100 000 per year. Mean age at time of bleeding was 52.7 years. In 36 patients the cause of the bleeding was an intracranial aneurysm; most of the aneurysms were localized to the anterior communicating artery and middle cerebral artery. In seven patients the cause was arteriovenous malformation. 23 patients (30%) died because of their subarachnoid haemorrhage; 15 from the primary bleeding and eight because of re-bleeding. The mortality for patients aged over 60 years was 48%, and for patients younger than 60 years 19%. There was a strict correlation between the initial clinical condition (Hunt & Hess scale) and the final outcome (Glasgow Outcome Scale).