10 patients with isolated or predominant mitral stenosis (mean mitral valve area 0.9 cm) and high preoperative values of pulmonary vascular resistance (mean 7.2 Wood units) were chosen for further hemodynamic postoperation evaluation. The patients had normal function of the aortic valve and no significant stenosis of coronary arteries. Hemodynamic evaluation was done by floating Swan-Ganz catheter preoperatively at rest and 6 months after mitral valve replacement at rest and during bicycle cycloergometer test in the supine position. Significant improvement in NYHA class was noted. Preoperatively 6 patients were in NYHA III class, 4 in NYHA IV class. After mitral valve replacement 4 patients were in class III and 6 in class II. Before mitral valve replacement only 3 patients were able to perform 25 Watt test. After surgery all except one performed at least 25 W. There was a significant decrease of pulmonary vascular resistance from 7.2 +/- 2.2 preoperatively to 3.1 +/- 1.9 Wood units at rest after the operation. Post operation during exercise pulmonary vascular resistance attained 4.2 +/- 1.9 Wood units. These changes were due to a fall in mean pulmonary artery pressure 49 +/- 13 at rest preoperatively to 25 +/- 6 mm Hg (rest) and 43 +/- 14 mm Hg (23 W) postoperatively and a rise in stroke volume index from 24 +/- 7 ml (m2) beat preoperatively to 32 +/- 9 at rest and 33 +/- 6 (25 W) postoperatively. Despite clinical and hemodynamic improvement there was no full recovery of pulmonary hemodynamics.