Silent mitral incompetence due to partial dislocation of the Starr-Edwards valve, leading to an apparently unexplained cardiac failure, has been observed in two patients, one two months and the other 35 months postoperatively. There was a fatal outcome in the first patient, no operation being carried out. Cardiac catheterisation with arteriography of the left anterior descending artery established the diagnosis, and led to a successful reoperation in the second case. It is therefore essential to carry out a haemodynamic and angiocardiographic investigation of any case who, after an initial trouble-free interval post-operatively, develops a deterioration of function which cannot be explained.