Posterior circulation aneurysm s, that account for approxim ately 15% of intracranial aneurysm s, 1 were am ongst the last to be treated successfully.The com m onest site for aneurysm form ation on the posterior circulation is at the basilar bifurcation and is closely follow ed by the origin of the superior cerebellar artery from the basilar artery (SC A). 2 T he next m ost frequent site is the origin of the posterior inferior cerebellar artery from the vertebral artery (PIC A aneurysm s). Aneurysm s arising on the posterior cerebral arteries, the basilar origin and the anterior inferior cerebellar artery (AIC A) are far less com m on. 2 Until comparatively recently, m any surgeons regarded som e, if not all, posterior circulation aneurysm s as being beyond direct surgical treatment. T his w as largely due to their inaccessibility, as w ell as the com plex and often variable anatom y of the vertebrobasilar system especially that of the perforating arteries. In addition, the relative rarity of these aneurysm s did not provide m any surgeons w ith an opportunity to gain suf® cient exp erience of their m anagem ent.The pioneering work of Jam ieson in Australia 3 and D rake in Canada 4 demonstrated that it was technically possible to attack these lesions, but these early attem pts at direct surgery were associated w ith a high m orbidity and m ortality. In D rake' s series of 14 cases published in 1965, 4 seven patients w ith basilar bifurcation aneurysm s died postoperatively. In this paper, D rake emphasized the crucial im portance of identifying and preserving the thalamoperforating arteries if the operation were to be successful.Subarachnoid haem orrhage is the m ost comm on m ode of presentation with posterior circulation aneurysm s. Loss of consciousness is said to be m ore com m only observed than with anterior circulation lesions. 1 Isolated cranial nerve palsies m ay result because of com pression of the relevant nerve by the aneu-rysm sac. 5 SCA aneurysm s m ay result in an ocular m otor palsy whereas PICA aneurysm m ay be related to the hypoglossal nerve. Large aneurysm sacs can cause brain stem com pression with long tract signs and even hydrocephalus due to obstruction of the cerebral aqueduct. 2,6 Basilar tip aneurysm s however, particularly those associated w ith a high basilar bifurcation m ay cause hydrocephalus by obstruction of the outflow of the third ventricle. W eber' s syndrom e can occur if the sac of the basilar tip or a SCA aneurysm com presses the crus cerebri and third nerve.Investigation of those presenting w ith subarachnoid haem orrhage is no different to those suspected of having anterior circulation aneurysm s. G ood quality CT scans should be obtained and if blood is not seen in the basal cisterns, lumbar puncture should be perform ed. Cerebral angiography with visualization of the origins of both PIC As is essential. T he pattern of bleeding as seen on C T m ay provide clues as to where the aneurysm is located which can be of particular value in cases of m ultiple aneurysm s. H owev...