One hundred consecutive patients with subarachnoid haemorrhage were investigated by computed tomography (CT) within five days of the bleeding. In 19 cases CT indicated other sources than a ruptured aneurysm, most often a cerebellar haematoma. The other 81 patients did not all undergo angiography. It could be estimated by extrapolation that an aneurysm would be demonstrated in 66 of the 68 patients in whom CST showed extravasated blood in the basal cisterns, and in only five of the remaining 13 patients. In a consecutive series of 50 patients with bleeding from a verified aneurysm, CT showed some extravasated blood in all cases and a haematoma in half of these. In 74% the site of the haemorrhage correctly predicted the site of the ruptured aneurysm. Intravenous contrast medium visualized the aneurysm on CT in 15 of 25 cases; this can improve the identification of the bleeding aneurysm to 88%. Rebleeding occurred in 21 patients, and was obvious in Ct in 19. If available, CT should be the first investigation in patients with first and recurrent subarachnoid haemorrhage.