Forty-five patients with aneurysms of the anterior cerebral artery distal to the anterior communicating artery were operated on by a direct approach method in the years 1960-1973. The incidence of aneurysms in this location was 4.8% of the total 1,000 aneurysms. It is of upmost importance in the treatment of aneurysms to insure the parent artery for the purpose of temporary occlusion. This makes it easier and safer to approach the aneurysmal neck and to handle possible premature aneurysmal rupture. From this technical standpoint, the aneurysms in this location were classified into two types, ascending and horizontal. Aneurysms of the pericallosal artery between the origin of the anterior communicating artery and the knee of the corpus callosum were designated as the aneurysms of the ascending portion, whereas the aneurysms of the pericallosal artery from the knee of the corpus callosum and beyond were designated as the aneurysms of the horizontal portion. Depending on the location of the aneurysm, craniotomy was performed at one of two different sites. For aneurysms of the ascending portion, bifrontal craniotomy was determined and applied as the safest approach. A small parasagittal craniotomy was determined to be sufficient for aneurysms of the horizontal portion. Although the total operative mortality was 4 of 45 cases (9.0%), no mortalities nor morbidities occurred in the last 12 cases since 1972, when the sites of craniotomies were differenciated.