The present paper is concerned with the 55 aneurysms out of a total of 158 that caused isolated paralysis of the oculomotor nerve. The majority arose from the internal carotid artery after it had pierced the dura (supraclinoid). Rarely the aneurysm sprang from the basilar artery. In two-thirds of the cases there had been a subarachnoid hæmorrhage from leakage. Not more than 10% of patients had arteriosclerosis. Calcification of the sac is not a sign that the aneurysm has thrombosed. The only certain way of demonstrating the position and size of an intracranial aneurysm is by arteriography, which is a safe procedure. The correct treatment is by carotid ligature. In about 8% of normals the circle of Willis is incomplete, therefore percutaneous compression must first be tried. The only fatalities from ligature were in persons in the acute stage of subarachnoid hæmorrhage, not from meningeal bleeding alone. In this type of case a clip applied to the neck of the sac is probably a better method. In the more usual cases where the hæmorrhage has been spontaneously arrested common carotid ligature in the neck is probably a little safer than intracranial clipping.