A new catheter with a releasable balloon has been developed for the controllable embolization of intracranial vascular lesions located beyond many arterial curves such as the carotid siphon. Embolization is accomplished by introduction of the balloon to the lesion, followed by inflating the balloon with a polymerizable liquid. After this liquid sets to a gel, the catheter is twisted off to detach the balloon at a specially designed joint. This balloon catheter has been used clinically to embolize an aneurysm of the middle cerebral artery.