SINCE THE impressive report by Starr and Edwards1in 1961 of initial clinical experiences with their ball-valve prosthesis, it has been widely and successfully employed.2-5Many patients have been successfully rehabilitated from severe congestive heart failure. The main limitation of the prosthesis has been the development of peripheral emboli in some patients from clots forming on the prosthesis. Anticoagulant therapy has apparently decreased the frequency of this complication, though exact data are not yet available. This report describes a very rare complication from a ball-valve prosthesis: 20 months after operation a proliferating clot produced acute obstruction of the mitral valve. The timely report by Garamella and associates6of a similar case with a fatal outcome, which was published a few months before this patient was seen, made it possible to establish a correct diagnosis and successfully replace the thrombosed prosthesis. Report of a Case A 25-year-old white woman