COMPLETE occlusion of the basilar artery has for many years been generally accepted as fatal or productive of severe neurological deficit. This concept stems principally from the fact that published case reports have been obtained almost entirely from postmortem studies and correlated retrospectively with the clinical manifestations. In this report of living patients in whom basilar artery occlusion was considered to have been demonstrated arteriographically, the extent of neurological deficit is shown to be dependent upon the adequacy of collateral circulation. When collateral pathways are available and patent, maintenance of function is possible. Cervicocranial panarteriography has made it possible to demonstrate not only normal patterns of blood flow in living patients but also the collateral pathways which bypass arterial occlusion. Information regarding the pathological involvement of the arterial tree and the effectiveness of collateral circulation will assist in planning therapy and in predicting the outcome following occlusion. For accurate localization