Pressure controlled ventilation has been proposed to recruit closed alveolar units and improve oxygenation through changing the inspiratory flow pattern from a square wave as used with volume controlled ventilation to a rapidly exponentially decaying curve and through maintaining airway pressure at a constant level throughout the inspiratory phase. The purpose of this study was to evaluate the cardiorespiratory efficacy of pressure controlled ventilation in severe respiratory failure. Cardiorespiratory values were measured in ten patients with severe respiratory failure on volume controlled and pressure controlled ventilation. Tidal volume, ventilatory rate, PEEP, inspiratory:expiratory ratio and FIO2 were maintained at the same level for both ventilatory modalities. Changing from volume controlled ventilation to pressure controlled ventilation was associated with significant improvement in PaO2 and decrease in peak inspiratory pressure. There were no significant changes in other cardiorespiratory values, such as arterial blood pressure, heart rate, inspiratory pause pressure and static compliance. These results suggest that pressure controlled ventilation may be a beneficial ventilatory modality in the treatment of severe respiratory failure compared to volume controlled ventilation.