The use of mechanical ventilation in prone position was proposed 20 years ago. Since then several investigations have been made trying to explain the mechanism whereby oxygenation is improved when the patient with ARDS is turned into the prone position. In supine position the lung perfusion is highest in the dorsal regions in normal healthy persons. However, when patients with ARDS are turned prone, the predominant dorsal perfusion is partly preserved, while at the same time the dorsal atelectases partly resolve, thereby improving the ventilation/perfusion ratio. Sixty-five percent of patients with early ARDS will achieve significant improvement in oxygenation in prone position. It is recommended that patients with early ARDS that remain hypoxic on mechanical ventilation with PEEP and inspiratory oxygen concentration above 60% be turned prone for 3-6 hours. If effective, the treatment can be repeated once or twice daily until regression of hypoxia. The possible effects of the prone position in other types of acute lung failure are so far not known.