Isolated aneurysms of the posterior spinal artery (not associated with arteriovenous malformations) are exceptionally rare. Three cases have been reported in the literature to date. We report a case of an isolated posterior spinal artery aneurysm causing acute subarachnoidal haemorrhage. Spinal artery aneurysms are contrasted with the more common intracranial aneurysms in terms of presentation and pathogenesis.