15% of patients with spontaneous subarachnoid haemorrhage have normal cerebral angiograms; they fare better than patients with demonstrated aneurysms, though rebleeding and cerebral ischaemia can still occur. In patients with a normal angiogram and accumulation of blood in the cisterns around the midbrain--"perimesencephalic nonaneurysmal haemorrhage"--outcome is excellent. To test the hypothesis that rebleeding and disability in angiogram-negative subarachnoid haemorrhage might be limited to those with other patterns of haemorrhage on initial computed tomography (CT), complications and long-term outcome were studied in 113 patients with angiogram-negative subarachnoid haemorrhage, admitted between January, 1983, and July, 1990. All patients were investigated with third-generation CT scans within 72 h of the event, and with cerebral angiography. The mean follow-up period was 45 (range 6-96) months. None of 77 patients with a perimesencephalic pattern of haemorrhage on CT died or was left disabled as a result of the haemorrhage (0% [95% confidence interval 0-5%]). Among the other 36 patients, who had a blood distribution on CT indistinguishable from that in proven aneurysmal bleeds, 4 had rebleeds and 9 died or were left disabled as result of the haemorrhage (25% [14-43%]). Thus, two distinct subsets of patients with angiogram-negative subarachnoid haemorrhage should be recognised. Patients with a perimesencephalic pattern of haemorrhage have an excellent prognosis. Rebleeding, cerebral ischaemia, and residual disability occur exclusively in patients with aneurysmal patterns of haemorrhage on initial CT. Repeated angiography in search of an occult aneurysm is justified only in the patients with aneurysmal patterns.