In 52 cases with 54 aneurysms of the anterior communicating artery (ACoA), the configurations of the ACoA complex and the aneurysm as observed during surgery were defined and the ability of pre-operative angiography to show the direction of fundus projection and the presence of anomalies was investigated. The aneurysms were either designated as being complex (9 %) or assigned to one of the primary positions: anterior (9 %). posterior (39 %). inferior (13 %). or superior (25 %). Marked hypoplasia of the Al segment (13 cases). fenestrated ACoA (7 cases). azygous A2 (1 case) and median callosal artery (1 case) were the anomalies that were observed. At angiography the anomalies could not be detected, but the direction of fundus projection could be determined correctly in 78 % of the cases.