Occlusion of an "accessory" distal anterior cerebral artery during treatment of anterior communicating artery aneurysms. Report of two cases. Two cases are presented in which clip occlusion of a third distal anterior cerebral artery segment occurred during treatment of anterior communicating artery aneurysms. Case histories, angiograms, operative descriptions, and postmortem findings are presented. The incidence of this anomalous vessel is reviewed. Preoperative and intraoperative vigilance in determining the presence of this anomaly prior to clip placement is emphasized. 