The pulmonary diffusing capacity (DLCO – steady state method according to Bates and coworkers) was measured at the time of heart catheterization in 12 patients with mitral stenosis without mitral incompetence. DLCO correlates with the tidal volume at rest and during exercise and with pulmonary vascular resistance during exercise only. DLCO and left atrial pressure exhibit a positive correlation up to 22.5 mm Hg only. In patients with mitral stenosis DLCO depends on the alveolar surface area and DLCO is influenced by regional changes in the pulmonary vascular resistanc