Influences of various factors on postoperative outcome of 122 well-documented cases, out of a total of 151 cases of middle cerebral artery aneurysm, are discussed in this paper. The shape of the aneurysm, major axis, major axis/neck and major/minor axis ratio were correlated with the presence or absence of a bleb (p less than 0.0000001), and indicated a correlation not only between aneurysmal shape and the outcome (p less than 0.05) but also between preoperative seriousness and the outcome (p less than 0.05). The preoperative seriousness and the grade of subarachnoid haemorrhage indicated a close correlation between each of these two parameters and the outcome (p less than 0.000001 for each). Both the quantity and the colour of CSF outflow from the chiasmal cistern showed close correlation with vasospasm (p less than 0.01), onset of hydrocephalus (p less than 0.01) and the outcome (p less than 0.001). Moreover, vasospasm and hydrocephalus grade itself also showed close correlation with the outcome (p less than 0.001 and p less than 0.00001). All parameters: blood pressure grade, BPmax, BPmin, BP fluctuation showed correlations not only with preoperative seriousness (p less than 0.001) and the outcome (p less than 0.0001) but also with subarachnoid haemorrhage frequency, number of aneurysms, and major axis/neck ratio.