We studied the hemodynamics of 14 patients with a Medtronic-Hall prosthesis in mitral position 6 months after valve implantation. The usual hemodynamic parameters showed a striking improvement after surgery. The mean pulmonary arterial pressure decreased from (mean +/- SD) 37 +/- 10.7 mmHg to 20 +/- 6.5 mmHg (p less than 0.005). Mean pulmonary capillary pressure decreased from 24 +/- 7.1 mmHg to 10 +/- 3.0 mmHg (p less than 0.001). Cardiac output increased slightly from 3.7 +/- 0.8 L/min to 4.2 +/- 0.45 L/min, the difference not being significant. Valvular area at rest was 1.82 +/- 0.25 cm2 and increased significantly during exercise to 2.47 +/- 0.45 cm2 (p less than 0.001). The opening angle of the disk occluder reached a mean value of 64 +/- 4.6 degrees, that is 91% of the maximal theoretical opening angle. We fitted a hydraulic function gradient = Ax (Flow)n (mean r = 0.9176), to the paired data of gradient and flow obtained during progressive exercise. This procedure permitted us to establish the actual hemodynamic behavior of the prostheses which did not concur with the behavior predicted by the Gorlin formula. We conclude that the Medtronic-Hall mitral prosthesis is an acceptable alternative when mitral valve replacement is considered.