We experienced 367 anterior communicating artery aneurysms (ACOAA) in 360 patients out of 1088 cerebral aneurysms operated on during recent 14 years. Three hundred and thirty three cases presented with subarachnoid hemorrhage, while the other 27 cases with unruptured ACOAA were detected during angiography for other ruptured aneurysm or other disease. The most common direction of ACOAA was anterior (39%), followed by superior, inferior, and posterior. The size of ACOAA was less than 10 mm in 303 (82/6%), between 11 mm and 24 mm in 59 (16.1%), and giant type in 5 (1.3%). Intraoperatively we found abnormalities of anterior communicating artery (ACOA) complex frequently. The vascular anomalies of ACOA complex were classified into four categories : anomalies of the A1, the ACOA, the A2, and the recurrent artery of Heubner. In 360 cases of ACOAA, A1, anomaly was the most common. Hypoplasia of A1 was observed in 58 cases (16.1%), aplasia of A1 in 15 cases (4.2%), and fenestration of A1 in 2 cases (0.6%). Duplication of ACOCA was present in 8 cases (2.2%), triplication of ACOA in 2 cases (0.6%), and fenestration/ plexiform of AOCA in 25 cases (6.9%). Unpaired A2 was shown in 4 cases (1.1%), triplication of A2 in 9 cases (2.5%), and frontal branch from A, in 2 cases (0.6%). The only anomaly of recurrent artery of Heubner was duplication, which was present in 10 cases (2.8%). In conclusion, for the successful surgery of ACOAA, we must take these anomalies into consideration in planning the surgical strategy.