OBJECTIVE To develop a method for reducing mitral regurgitation in dogs by positioning and tightening a circumferential suture around the mitral valve annulus. METHODS Description of clinical cases. METHODS 15 dogs with spontaneous mitral valve disease, annular dilation, and refractory congestive heart failure treated between 1962 and 1994. METHODS A composite suture material was constructed using braided Teflon-impregnated polyester sternotomy suture, barium sulfate thread, and Silastic tubing. Through a left lateral thoracotomy, the suture was positioned around the mitral annulus by passing it through the coronary sinus, right atrium, and underneath the left coronary arteries before tightening it. RESULTS Six dogs died during surgery because of hemorrhage from the left atrium, coronary sinus, or left coronary artery. Three dogs died postoperatively because of coronary artery compression by the suture. Satisfactory suture placement was achieved in six dogs, three of which were long-term survivors (6 to 26 months) before they were euthanatized for noncardiac reasons. CONCLUSIONS Reduction of mitral annulus diameter with a circumferential purse-string suture does not require cardiopulmonary bypass. The surgery is technically difficult, and detailed knowledge of coronary artery anatomy is required. The current suture design is inexpensive, radiopaque, and biocompatible; no special instruments are required for placement. CONCLUSIONS Mitral regurgitation is the most common cardiovascular disorder in dogs and the most common cause of congestive heart failure. Annular dilation occurs as a consequence of mitral regurgitation. Placement and tightening of a circumferential suture around the mitral annulus reduces the degree of mitral regurgitation.