Background: One of the most characteristic findings of cardiovascular response to exercise in mitral stenosis is the inappropriate increase in pulmonary artery pressure which has been known to be a major limiting factor for exercise capacity. We sought to investigate whether sildenafil, selective phosphodiesterase-5 inhibitor can reduce exaggerated increase in pulmonary artery pressure and improve exercise tolerance in patients with mitral stenosis. Methods: Twenty four patients (19 females, 5 males, mean age 53.4 짹 10.1 years) with moderate to severe mitral stenosis were studied. In all patients, supine bicycle exercise echocardiography was performed before and 60 minutes after 50 mg of sildenafil intake with simultaneous measurement of indices of cardiopulmonary exercise test. Results: There were no changes in blood pressure, heart rate, arterial oxygen saturation, and transmitral mean pressure gradient before and after sildenafil. There was a significant reduction in peak pulmonary artery pressure (from 72.4 짹 15.3 to 62.0 짹 12.5 mmHg, p < 0.001), pulmonary vascular resistance (from 0.22 짹 0.09 to 0.19 짹 0.07, p=0.004) defined as the ratio of TRV (tricuspid regurgitant velocity) to TVI_RVOT (right ventricular outflow tract time-velocity integral), and an increase in cardiac output (from 5.0 짹 2.8 to 5.3 짹 2.9 L/min, p=0.11). Furthermore, there was a significant increase in exercise duration (from 403 짹 115 to 442 짹 115 sec, p=0.016 ), VO2 max (from 15.3 짹 6.5 to 18.3 짹 4.9 ml쨌min^-1쨌kg^-1, p=0.016), AT/VO2 max (from 17.5 짹 9.9 to 23.6 짹 12.6 %, p=0.014), and a decrease in VE/VCO2 slope (from 91.0 짹 42.6 to 73.2 짹 29.1, p=0.03) (Table 1). Conclusions: Sildenafil improves exercise tolerance by modulating pulmonary artery pressure and pulmonary vascular resistance in patients with mitral stenosis.