Angiographic examinations were made of 9 patients who had complete occlusion of the basilar artery. Six are alive 3 months to 5 years after diagnosis. In 7 of the patients the diagnosis was not suspected prior to angiography. The commonest site of occlusion was just distal to the origins of the posterior inferior cerebellar arteries. Various patterns of collateral flow were observed. A direct relationship is indicated between patency of collateral pathways and prognosis.