The hemodynamic response to controlled oxygen administration was investigated in 35 patients who presented with acute decompensation of chronic obstructive pulmonary disease. Gasometric and hemodynamic data were successively measured under inspiratory fractions in oxygen of 0.21 (room air) and 0.28. Response to oxygen therapy was characterized by a decrease in cardiac output and an increase in oxygen delivery because of a sharp increase in arterial oxygen content. The coefficient of oxygen delivery (oxygen delivery/oxygen consumption) increased in each patient. All these changes were highly significant (p les than 0.001). Somewhat unexpectedly, there was no change in pulmonary artery pressures, so that pulmonary arteriolar resistance even increased (p less than 0.05). Further analysis individualized 2 types of hemodynamic response to oxygen therapy. In 15 patients who presented with severe hypoxemia, oxygen therapy resulted in a significant increase in oxygen delivery without change in cardiac output. By contrast, in 20 patients with moderate hypoxemia, oxygen therapy induced a significant decrease in cardiac output (p less than 0.001) without change in oxygen delivery. Patients in whom cardiac output was unaffected by oxygen administration presented with a lower mixed venous oxygen tension (less than 35 mmHg) and a lower coefficient of oxygen delivery (less than 3.5). This study demonstrated that oxygen therapy does not always increase oxygen delivery in acute decompensation of chronic obstructive pulmonary disease.