Postoperative hemodynamic studies have been performed on 11 subjects following replacement of the mitral valve. In eight cases a comparison was made with the preoperative findings.The great improvement in the functional capacity of most patients is reflected in the findings of this study, which indicate a small gradient at rest across the prosthesis and no mitral incompetence. The residual gradient was thought to be due mainly to inadequate orificial size, although delay in reaching the fully open position may also play a part.In two subjects a severely elevated pulmonary vascular resistance fell markedly following the relief of left atrial hypertension.Two patients failed to show sustained clinical and hemodynamic improvement. In one, mild aortic valve disease was present and severe systemic hypertension developed, resulting in death. The influence of these factors is discussed. In the other, the cause for the continuing heart failure with poor left ventricular contractility was undetermined.