Clinical characteristics and the outcomes of 50 patients with confirmed subarachnoid haemorrhage (SAH) of unknown aetiology were evaluated. Twenty-nine of these patients were interviewed with a mean follow-up of 7.7 years. Seven patients experienced early rebleeding during hospitalization, and one patient experienced late rebleeding during the follow-up period. On admission to the hospital the consciousness level was "clear" in 43 of the patients, "drowsy" in 6, and "semicomatose" in only 1. Of 10 patients who underwent computerized tomography (CT) within 3 days after SAH, 6 showed no abnormal CT findings and 3 had mild subarachnoid blood. Another patient showed severe SAH. Neurological deficit due to symptomatic vasospasm persisted in only 1 case. Two patients died after re-bleeding during hospitalization, and two others died during the follow-up, thereby representing an overall mortality rate of 8%. Of the latter cases, one died of an unrelated disease, and the other died suddenly from an unknown cause. The remaining forty-six patients (92%) had a good outcome. These results confirm previously reported clinical characteristics, namely that the symptoms of the majority of such cases are mild, the incidence of symptomatic vasospasm is rare, the risk of late re-bleeding is very low (less than 1% per year), and the prognosis is good. The CT findings suggest that some of these features resulted from only mild SAH. Thus, SAH of unknown aetiology seems to be a disease entity with a good prognosis.