The investigation was carried out on arterial anomalies complicated with 296 cases of single aneurysm of anterior communicating artery and on the etiological significance of the anomalies in the development of cerebral aneurysms. Either the fenestration in anterior communicating artery or the presence of more than 2 anterior communicating arteries was observed in 17 cases (5.7%). As the anomaly was duplicated in some of these cases, the total number of cases with anomalies of anterior communicating artery became to be 26 cases (8.8%) out of 296 cases investigated. This occurrence rate was not higher than those observed in other cerebral aneurysmal cases and the control cases, which were reported previously. There was no significant difference as to the occurrence rate of hypoplasia of the A1 portion between the cases whose aneurysms of anterior communicating artery were complicated with arterial anomalies and the cases without the arterial anomalies.