Ventilatory response to CO2 was examined in 6 patients with severe chronic obstructive lung disease (COLD) shortly after discontinuance of long-term ventilation for acute respiratory failure (ARF). Ventilatory response to CO2 was extremely low in all patients 6-10 h after discontinuance from ventilator. Our month later, however, CO2 response curves shifted to the left and in 3 patients there was improvement which was seen in the slope of response curve. In patients who had mechanical ventilation for ARF from COLD, abnormally low responses to CO2 probably contribute to the difficulty in weaning from mechanical ventilation. The presence of reduced CO2 responsiveness should be considered in these circumstances.