We evaluated the therapeutic effect of percutaneous transluminal mitral commissurotomy (PTMC) on pulmonary function, and the damage caused to pulmonary vasculature by transient occlusion of the mitral valve during PTMC in patients with mild mitral stenosis. Pulmonary function tests were done and serum thrombomodulin was measured in 6 patients before and after PTMC. The mean pulmonary arterial pressure and mean pulmonary arterial wedge pressure decreased significantly from the control values, and the decreases were proportional to the increase in the area of the mitral valve. VC, %VC, and DLco were in the normal range before and after PTMC but PEFR, FEV1%, FEV1, V50, V25, and V50/V25 were significantly higher after PTMC. Change in the area of the mitral valve correlated with the changes in FEV1% (r = 0.841), in V50 (r = 0.624), and in V25 (r = 0.697). The serum thrombomodulin levels before and after PTMC did not differ. We conclude that pulmonary dysfunction in patients with mild mitral stenosis MS is mainly due to an obstructive ventilatory defect, and that PTMC can correct this dysfunction without damaging the pulmonary vasculature.