Of 274 operated cases of cerebral aneurysm, temporary occlusion of the cerebral arteries was used in 215 cases (79%). Of the 215 cases, 177 (82%) showed no sequelae. The maximum safe time limit for temporary occlusion at the bilateral A1 portion of the anterior cerebral arteries was 48.5 min at 26 degrees C, 47 min at 27 degrees C, and 42 min at 28 degrees C; at the M1 portion of the middle cerebral arteries, it was 30 min at 26 degrees C, 35 min at 27 degrees C, 36 min at 27.5 degrees C, 40 min at 30 degrees C and 19 min at normothermia; and at the dominant A1 portion, in cases of hypoplasia at the contralateral A1 portion, it was 82 min at 26 degrees C, 86 min at 27.5 degrees C, and 63 min at 28 degrees C. Consequently, when performing direct surgery on aneurysms of the anterior communicating artery, unilateral clamping of the A1 portion prevents rupture during surgery and has the advantage of prolonging the occlusion time. Neurological sequelae may have been caused by the temporary occlusion of the cerebral artery in one case, and by the temporary occlusion or the surgical operation in six cases. Of the 22 fatal cases, only one was thought to be due to the temporary occlusion of the cerebral artery. Intermittent release of the clamping for 5 to 10 min is considered to be effective in prolonging the safe time limit of temporary clamping of the cerebral arteries in surgery of cerebral aneurysm under moderate hypothermia.