Carotid ligation was performed in a series of 30 consecutive cases of infraclinoid aneurysms, of which 6 had not ruptured, and 9 cases of supraclinoid aneurysms. Fifteen of the ruptured aneurysms were operated on before, and 18 after, the eleventh day after bleeding. The internal carotid artery was primarily ligated in four cases, with two deaths, and in one case lasting hemiparesis occurred. In the rest of the cases, the common carotid artery was primarily ligated without operative mortality. After varying intervals, the internal carotid artery was ligated both in the neck and intracranially in 25 cases. In three cases late complications occurred due to embolus and thrombosis, resulting in one death. In four cases there were transient symptoms caused by insufficient cerebral circulation. The cerebral circulation was investigated in 23 cases using an intravenous isotope technique before and after ligation of the common carotid artery. In each of the cases the circulatory values were sufficient, with a slight diminishing of the flow on the ligated side. After ligation of the internal carotid artery at a later stage the flow in both hemispheres increased. This finding has been interpreted as being due to the interruption by internal carotid ligation of the retrograde internal carotid flow that occurs after common carotid ligation. In five cases aorto-cervical angiography was performed some months after ligation of the common carotid artery, and in four a retrograde flow in the internal carotid artery on the ligated side was shown. In accordance with the results obtained suggestions are made for carotid ligation in the treatment of carotid aneurysms not accessible for neck ligation.