This study concerns 64 patients with angiographically negative subarachnoid haemorrhage (SAH) hospitalized in the period 1970-1982. Requisites for inclusion in the study were adequate angiographic demonstration of the carotid and vertebrobasilar systems and no clinical signs of spinal SAH or spontaneous intracerebral haematoma. The clinical data on the 64 cases confirm the close similarity, except for the prognostic factors, between angiographically negative SAH and SAH secondary to rupture of an intracranial saccular aneurysm. The study underlines the benign character of the clinical course and of the medium and long-term prognosis of the condition under study. In view of this, the hypothesis advanced sometime ago relating angiographically negative SAH to the rupture of microaneurysms (phi less than 2 mm) of the large cerebral arteries with subsequent complete repair of the artery wall, or to the spontaneous thrombosis of intracranial saccular aneurysms, with the possibility of subsequent recanalization and risk of fresh rupture, would appear to be a reasonable one.