The haemodynamic status after valve replacement surgery and the preoperative haemodynamic parameters have been studied for their predictive value in a series of 217 patients who have had aortic, mitral or mitro-aortic valve replacements; all cases had a preoperative catheterisation, and 132 had a postoperative catheterisation at an average of 15 months postoperative. The following conclusions have been drawn from the study: 1. The operation (which consisted of replacement with a Starr valve with Silastein ball-valve in 8 cases out of 10) resulted in a substantial, and statistically significant, improvement in the minor circulation pressures and in the cardiac index. The ejection fraction also increases in most cases. 2. The increase in the cardiac index is significantly greater in aortic stenosis than in aortic incompetence, and in the aortic group than in the mitral group. 3. Improvement is early, and occurs within the first few weeks, in the aortic group; it occurs more gradually in the mitral group, in whom serial catheterisations show a gradual decrease in the pulmonary arterial and capillary pressure and an increase in the cardiac index which may be progressive until the second postoperative year. 4. Analysis of the mean values of the various parameters as a function of the clinical findings, and of the incidence of failures and early or late deaths from haemodynamic changes, has failed to show that the preoperative haemodynamic picture has a statistically significant predictive value.