Abstract: An adjustable ringprosthesis designed particularly for being sewed onto the ring of the mitral valve or tricuspidal valve of a living heart uses a spring ribbon coiled with an overlap of about a half turn covered in a woven, braided or knitted cuff of a suitable synthetic fiber that can be sewed into place. An adjusting device (40) is fixedly positioned on a portion of the spring ribbon which is internally overlapped by an end portion which carries teeth so as to form a rack which can be driven by a screw (15) in the adjusting device for varying the diameter of the ring. The adjusting device is operated by an adjusting tool, the front end of which is guided into a screw-head socket (50) for turning the screw to change the ring diameter. The shaft of the tool is guided by a tube through a connecting piece (51) onto which the guide tube is screwed, and from which it can be unscrewed and removed when there is no further need for post-operative adjustment of the prosthesis.