Using an animal model of acute respiratory failure (ARF), we evaluated two treatments: conventional mechanical pulmonary ventilation (MV) and continuous positive airway pressure (CPAP) with extracorporeal removal of CO2 by an artificial membrane lung. We developed a model of 'mild' ARF and a model of 'severe' ARF after ventilating healthy sheep at a peak inspiratory pressure of 50 cm H2O for various lengths of time. Sheep from either injury models were randomly assigned to one of the above treatment groups. All 16 sheep from the model with 'severe' ARF died, with progressive deterioration in pulmonary function and multiorgan failure irrespective of the treatment. Of 11 sheep from the model with 'mild' ARF treated by MV, only three survived, whereas all 11 sheep from the model with 'mild' ARF treated with CPAP and extracorporeal removal of CO2 responded well, and nine sheep ultimately recovered. We conclude that CPAP with extracorporeal removal of CO2 provided a better environment for the recovery in our model with 'mild' ARF than the conventional arrangement centered on MV alone. Our studies also suggest that lung injury can progress (i.e., model with 'severe' ARF) to where neither of the two treatments can succeed.