A classification of aneurysms of the posterior cerebral artery from a series of 8 personal cases and 34 cases collected from the literature is attempted. The classification is based on the topography and sites of origin of the cortical and central branches of the artery. The artery was divided into six sections which permitted the description of the origin and projection of typical aneurysms. Single cases of atypical aneurysms can be explained by the known vascular anomalies. The predilective site is section B (origins of the posteromedial choroidal artery and quadrigeminal artery), then section D with the main division of the posterior cerebral artery and origins of the anterior temporal artery, the anterior posterolateral choroidal artery, the hippocampal artery and the thalamogeniculate artery, and finally section C--the junction with the posterior communicating artery. Clinical syndromes corresponding to these locations are described. The classification, when considered together with improved angiographic technique and microsurgery, allows exact preoperative and peroperative definition of topography which in turn enables the avoidance of injury to functionally important typical and atypical central branches of the posterior cerebral artery.