Eighty-six of 996 patients with primary subarachnoid haemorrhage (SAH) had negative pan-angiography studies. These 86 patients with subarachnoid haemorrhage of unknown origin (SAH-NUD) were compared with 853 patients sustaining an aneurysmal bleed (SAH-A) admitted during the same period 1980-1989. The age and sex distribution of both groups were similar. The SAH-NUD group was in better condition on admission, with less blood evident on CT scan. All 16 (repeat control) angiography studies in the SAH-NUD group were negative. During a follow-up period ranging from 1 to 10 years (mean 5.4 years), two patients experienced rebleeding with negative repeat angiographies and subsequent total recovery. Using the Glasgow Outcome Scale (GOS) the final outcome was good in 86% of the study group and 54% of the aneurysm group. However, half of the SAH-NUD patients complained of persistent symptoms at long-term follow-up. Thus, despite a generally good prognosis, for a given individual SAH-NUD may be catastrophic with many residual symptoms persisting for the rest of the person's life.