A review of 413 cases of middle cerebral artery aneurysms out of the means of 1621 cases which were operated upon by one of the authors (J. Suzuki), is made with regard to their surgical management. At operation, the patient is placed in the supine position with the head fixed neutral and chin slightly up. Small craniotomy is performed by means of a small semi-circular skin incision just behind the hair line. After exposure of the internal carotid artery, the arachnoid overlying the Sylvian fissure is cut distally in order to expose the middle cerebral artery. The most important principle for aneurysm surgery is to secure the afferent artery prior to aneurysm exposure. In our approach, the M1 portion runs horizontally so that we can secure the parent artery after evacuation of liquor at the chiasma and then dissect the aneurysm. We usually put the temporary clips on the afferent and efferent arteries of aneurysm, that is the M1 and M2's during dissection of the aneurysm. In order to prolong the time of temporary arterial occlusion, 20% mannitol solution is administered intravenously just prior to dural incision under normotensive and normothermic anesthesia. Under such condition, it is possible to occlude the middle cerebral artery for 40 minutes. On the treatment of aneurysm, it is important to expose the whole aneurysm. If only neck portion is exposed and treated, kinking or stenosis of the parent artery can occur due to adhesion, and a second aneurysm or small artery hidden behind can easily overlooked. Therefore, exposure of the entire aneurysm is necessary. Subsequently, ligation should be carried out, then neck clipping performed.(ABSTRACT TRUNCATED AT 250 WORDS)