Thirty-six patients with respiratory distress syndrome (RDS) were treated with mechanical ventilation with positive end-expiratory pressure (PEEP). In 16 patients the RDS was the result of direct pulmonary injury due to chest trauma (group A), and in 20 patients the RDS followed non-direct injury of the lungs (group B). Fifteen patients died. At the moment of death the RDS had disappeared. No difference in mortality existed between groups A and B. Four patients died from causes unrelated to the RDS, one patient died from an avoidable error in the treatment. The ultimate outcome did not seem to be related to age, use of mechanical ventilation or PEEP, or occurrence of complications (e.g. a pneumothorax, which appeared to be related rather to the chest trauma). Of probably prognostic significance was the presence of a combination of massive aspiration and RDS. Of the 12 patients who suffered from this combination, nine died. PEEP ventilation was more unsuccessful in raising PaO2 appreciably in these patients than in others. The combination of massive aspiration and RDS posses a therapeutic dilemma. PEEP ventilation is invaluable in the treatment of RDS. Interruption of the PEEP within the first 1-2 days may sometimes result in massive recurrence of the RDS. Endotracheal suctioning which is a prerequisite for the treatment of massive aspiration, may under these circumstances be virtually impossible within this period. A compromise, including PEEP ventilation for as short a period as possible (usually no longer than 2 to 3 days) without interruption, followed thereafter by intensive physiotherapy with endotracheal suctioning, may succeed in saving some of the patients.