Cloth-covered Starr-Edwards prosthesis, was used to replace either the aortic valve (54 cases) or the mitral valve (71 cases), between 1968 and 1978, in an attempt to reduce the incidence of thrombo-embolic complications. Comparative studies using a silastene ball prosthesis during the same period showed that after aortic replacement, the mortality rate during the first month (12 p. cent), late deaths (actuarial survival curves), and most complications (including thrombo-embolin) were not significantly different in the two groups. Hemolysis is significantly greater after the two groups. Hemolysis is significantly greater after replacement with the metallic ball prosthesis; in one patient there was a mechanical change in the material. After mitral replacement, hospital mortality rates (6 p. cent) and the survival curves, after silastene or cloth-covered prostheses, did not differ significantly. Cloth covered mitral valve cause a significantly higher, but compensated rate of hemolysis, a metallic noise heard by 22 p. cent of the patients, and a significant reduction (about 1/5) in the incidence of thromboembolic complications.