This report presents data obtained in the care of 830 patients requiring assisted ventilation. When these patients were divided into groups by the severity of their respiratory failure as defined by the duration of ventilatory assistance (>48 hours, <48 hours) and level of positive end expiratory pressure (PEEP) required (>5 cm HoH, <5 cm HoH), it was found that evidence of concurrent bacterial infection was present in the majority of patients with severe respiratory failure. This finding could not be explained by infection acquired after the onset of respiratory failure. In addition, this analysis demonstrated the important association of active pulmonary infection with the occurrence of barotrauma in these patients. Case analysis of patients subjected to extracorporeal membrane oxygenation has led to the suggestion that underlying sepsis in patients failing to respond to conventional ventilatory assistance similarly limits the usefulness of membrane oxygenator support.