The authors survey 443 cases of intracranial aneurysms treated in the past seven years. 403 cases were operated upon with microsurgical techniques. The operative mortality was 5.4 per cent, and 82.4 per cent of surgically treated cases are well and working, leading useful social lives. It was found that cases submitted to surgery in the first three days after subarachnoid haemorrhage (SAH) (the day of SAH being counted as the first day) showed good results, little appearance of postoperative vasospasm, and no mortality due to vasospasm. Cases operated upon after one week from the insult of SAH also showed good results, whereas fatal postoperative vasospasm was seen in cases operated upon on the 4th--7th day after SAH. Cisternal, ventricular, and epidural drainage are recommended after the clipping of aneurysms in the acute stage of SAH. There were 68 cases with preoperative vasospasm. There was no case in which vasospasm was identified during the first four days after SAH, while 66 per cent of the cases exhibited vasospasm between the sixth and ninth days after SAH. These 68 cases can be classified into four groups: 1. 8 cases died from vasospasm before surgery: 2. 8 cases had renewed bleeding mainly when vasospasm began to subside. 3. 22 cases underwent surgery after vasospasm had subsided, the duration of vasospasm ranging from 8 to 24 days, on an average 14 days; 4. 30 cases underwent surgery while vasospasm was still present; of this group, (4E) 15 cases submitted to surgery, on an average 4.5 days after the onset of vasospasm, manifested deterioration of clinical states because of aggravation or new appearance of vasospasm; (4L, 15 cases which underwent surgery, on an average 7.4 days after the onset of vasospasm, showed no such deterioration. In the follow-up, well and working cases were seen in 45.5 per cent (3.), 60 per cent (4E), and 80 per cent (4L), respectively. The authors classified vasospasm into three types: Type 1, extensive diffuse, Type 2, multi-segmental, and Type 3, local. Type 1 was prognostically worst, Type 3 good, and Type 2 was located between these two types.