Pulmonary diffusing capacity (measured by the steady-state method according to Bates) was measured in 18 normal subjects and in 40 patients both in supine and upright position; right heart catheterization was performed in 33 of the subjects. In normal subjects the mean value of DLCO was 26.6 ± 8.0 % lower in the upright than in the supine position. In patients with mitral stenosis and in patients after pneumonectomy the DLCO did not change with body posture. In patients with interstitial pulmonary fibrosis the DLCO was 20.6 ± 11.3 % lower in upright than in supine position. The postural changes in DLCO correlated with the pulmonary arterial pressure, left atrial pressure and with pulmonary vascular resistance. The value of DLCO is influenced by the lung volumes and by the quality of alveolocapillary membrane, the postural changes of the DLCO are the indicator of the level of the pulmonary arterial pressure. In patients with severe postcapillary pulmonary hypertension (50 mm Hg) the DLCO may be higher in upright than in supine positio