Our study of 100 multiple aneurysm cases show that 66 survivied the initial danger period either with surgery, or by conservative treatment. Of these only one may have died subsequently from bleeding from an incidental aneurysm, during a continuing follow-up period now averaging more than seven years. In a separate series of 75 patients presenting clinically with subarachnoid haemorrhage it was possible to lateralize or approximately localize the site of bleeding by CAT in 56; 31 were from aneurysms and 13 from arteriovenous malformations. In the remaining 19 cases CAT was unhelpful in indicating the site of haemorrhage; 12 had aneurysms; no cause was found in the remainder. CAT was more valuable in localization in the seven days following the haemorrhage (86 per cent) than later (34 per cent). The low risk of bleeding from incidental aneurysms makes us feel that treatment confined to the lesion which is responsible for the subarachnoid haemorrhage is not an unreasonable policy in the large majority of these patients. Detailed angiographic study of the site of haemorrhage is performed, but once this has been achieved, more extensive angiography is not routine, but is determined by the requirements of the surgery directed at the site of haemorrhage.