Gaseous exchange and pulmonary hemodynamic data were studied in 34 patients with chronic obstructive lung disease, during acute respiratory failure. Control tests were carried out in 20 patients after recovery. Pulmonary hemodynamic studies were performed by the microcatheterization method (floating catheter). On an average cardiac output was found normal during acute failure and it did not change significantly after recovery. During acute respiratory failure the mean PAP increased from 25.6 to 39.4 mm Hg (P < 0.001). Hypoxia accounts the best for this increase of the PAP : PAP is well correlated with the decrease of arterial PO2 during acute failure (r = 0.75; P < 0.001). The influence of hypercapnia and acidosis on pulmonary hemodynamics seems to be very small in our patients; likewise the influence of mechanical factors (elevated alveolar pressure) seems to be negligible. Contrary to some authors we have not found elevated pulmonary capillary pressures during acute respiratory failure.