Case materials and methods Between 1961 and 1980 we have encountered 55 cases of giant aneurysm, which are larger than 25 mm in diameter. Of these cases 27 are male and 28 female. They were between 15 and 69 years of age at the time of admission, the average age being 50.1. The aneurysms were located as follows: the internal carotid artery in 29 cases, the anterior communicating artery in 12, the basilar artery in seven, the middle cerebral artery in six and the anterior cerebral artery in one. Direct operation is performed in 32 cases (12 cases on the anterior communicating artery, eight on the internal carotid, six on the middle cerebral, five on the basilar and one on the anterior cerebral artery). The rate of direct operation is 58.2% (Table 1). As we routinely use temporary clips on the afferent and/or efferent arteries during aneurysmal dissection, hypothermic anaesthesia around 27 ° C was adopted before 1972 and normothermia with administration of 20% mannitol solution has been used since 1973 in order to protect against brain infarction. By the intravenous administration of 500 ml of 20% mannitol solution just before the temporary occlusion, we can stop the blood flow of any cerebral artery for 40 minutes at most without any danger or sequelae. Since the effectiveness of mannitol solution is limited to a period of 100 minutes after the administration, we have to give an additional 500 ml more when longer occlusion of the arteries is required. The method of direct operation is excision for 16 cases (nine on the anterior communicating, three on middle cerebral, three on the basilar and one on the anterior cerebral artery), neck ligation and clipping for 13 cases (eight on internal carotid, three on anterior communicating, one on the basilar and one on middle cerebral artery), trapping for two cases (one on basilar artery and one on the middle cerebral artery), and muscle wrapping for one case of a sclerotic middle cerebral aneurysm (Table 2). There were 23 aneurysms impossible for direct operation including 21 on the internal carotid artery and two on the basilar artery. Common carotid ligation was carried out in nine cases of internal carotid aneu-