Two obese patients with acute respiratory failure are described. Both required mechanical ventilation to reverse the respiratory acidosis. On recovery, the major abnormality was failure of ventilatory response to inhaled carbon dioxide. After oral progesterone (100 mg/day and 20 mg/day, respectively) the slope of the carbon dioxide response line increased from 0,02 to 1,14 I min-1 mmHg-1 and from 0,04 to 1,14 I min-1 mmHg-1, respectively. This was associated with sustained clinical remission in 1 patient.