We consider our method with an inflatable and releasable balloon to be trustworthy and now applicable to the treatment of carotido-cavernous fistulas; it may not be possible in all cases to pass the ballon on through the fistula and inflate it in the cavernous sinus, but it seems to us so very important to preserve the carotid flow that we think that this should be achieved whenever possible. Certain arterial aneurysms could profit from this technique. We have purposely omitted all diagnostic application of the method which serbinenko has described in his article: selective opacifcation of certain branches of the internal carotid, deliberate temporary occlusion of certain branches to enable the amount of tolerance to be assessed. All these exploratory methods which we are beginning to carry out will form the subject of a later paper.