A 20-year-old woman had received a 22-mm Lillehei-Kaster prosthesis at the age of 16 for progressive mitral valve stenosis. She was asymptomatic for 4 years, when dyspnea on exertion, orthopnea and paroxysmal nocturnal dyspnea recurred. Operation was undertaken and a thrombus of recent origin was found extending from the posterior aspect of the prosthetic ring to the posterior left atrial wall. The sewing skirt was covered with neoendothelium and the valve orifice was not compromised; however, the hinge mechanism on the ventricular surface was overgrown with a dense fibrotic pannus that limited the normal 80° excursion of the tilting disc to 15°. This marked limitation of disc motion produced the equivalent of severe mitral stenosis. The Lillehei-Kaster valve was excised and replaced with a #27 Björk-Shiley prosthesis. The patient improved, and she remains asymptomatic 1 year after surgery.