Aiming at investigation, a Beall's valve was implanted in mitral or in mitral-tricuspid positions only on 100 patients, between November 1969 and February 1972. The details of the initial valve lesions, the clinical condition of the patients, and the modalities of prosthetic replacement are described. The causes of death within the first month were analysed. None could be related with the prosthesis itself. At distance, 17 patients were lost of sight, 18 died and 51 were studied from 12-42 months after operation. It results from this investigation: -- that 84% of the patients reexamined were improved. Most of them were included in a better functional class; -- that the incidence of thrombo-embolic accidents amounted to 12% for the whole group of patients, a percentage similar to that observed with other types of prostheses; -- that in contraat, haemolysis is an obvious disadvantage of Beall's prosthesis (20 of 51 cases); -- that abnormal wears in two valves resulted in valve dysfunction compelled to re-operation. These latter two incidents have lead to abandoning this type of prosthesis.