Controlled oxygen therapy may aggravate carbon dioxide retention during acute exacerbations of chronic obstructive pulmonary disease (COPD). Of 50 consecutive patients with COPD and acute respiratory failure, 13 required intubation because of carbon dioxide narcosis. With discriminant analysis of their arterial oxygen tension (PaO2) and pH on admission, a diagram separated patients into those at high risk and those at low risk for carbon dioxide narcosis. This diagram was then used to predict carbon dioxide narcosis in 73 patients with COPD and acute respiratory failure who were treated with controlled oxygen. In 16 of these patients carbon dioxide narcosis developed. Thirteen (81 per cent) were predicted by the diagram to be at high risk for this complication. Only two (4 per cent) patients judged by the diagram to be at low risk for carbon dioxide narcosis required mechanical ventilation. Utilizing an oxygen tension (PO2), carbon dioxide tension (PCO2) diagram a patient's ventilatory response was compared to that of ambulatory patients with COPD. These data suggest that hypoxemia and acidosis are more discriminatory for "carbon dioxide narcosis" than hypercapnia.