OBJECTIVE: The anterior communicating artery(ACoA) is known to be the most frequent location of intracranial aneurysms, but the complex arterial anatomy of the ACoA region makes this aneurysm among the most difficult one to treat. In the treatment of ACoA aneurysms, the direction of aneurysmal fundus is known to be very important in the surgical tactics. All ACoA aneurysms in our series were classified according to its direction, and analyzed the clinical features in order to investigate the prognostic factors influencing upon the surgical outcome. METHODS: The authors reviewed 236 cases of ruptured ACoA aneurysms that were operated from 1990 to 1997, were classified according to Pia's classification. RESULTS: The incidence rate of the ACoA aneurysm was 35.1%(236/672). Ventral group was more common than dorsal group, especially in ventro-caudal projection subgroup(36.0%). Poor preoperative clinical grade(Hunt-Hess grade IV and V) patients were more common in dorsal group(13.1%) than ventral group(2.6%). Rebleeding and intracerebral hematoma were more commonly seen in ventral group. However, vasospasm, hydrocephalus, hyponatremia, and intraventricular hemorrhage were observed more frequently in dorsal group. Worse outcome was more common in dorsal group than ventral group, especially in dorso-caudal projection subgroup. Also, poor outcome was identified in patients with intracerebral hematoma, intraventricular hemorrhage, hyponatremia, and hypertension, although statistically insignificant. In cases with the A1 dominancy, there was no difference in surgical outcome between the right and left side approach. The higher the aneurysmal neck from the planum sphenoidale, the worse outcome via pterional approach. CONCLUSION: It seems that the preoperative clinical grade, aneurysmal direction, and the height of aneurysmal neck, especially in the pterional approach, would be the major prognostic factors, and that intracerebral hematoma, intraventricular hemorrhage, hyponatremia, hydrocephalus and the intraoperative aneurysmal rupture would be the minor prognostic factors.