To assess the potential benefits of the prone position for gas exchange in patients with acute respiratory failure, we turned 6 patients from supine to prone, supporting the upper thorax and pelvis and allowing the abdomen to protrude. Arterial PO2 increased by a mean of 69 mm Hg (range, 2 to 178 mm Hg) at the same tidal volume, same inspired oxygen concentration, and same level of positive end-expiratory pressure. The maneuver made it possible to reduce the inspired oxygen concentration in 4 of the 5 patients who required mechanical ventilation of the lungs and to defer intubation in the patient who was breathing spontaneously. After subsequent turns from supine to prone, arterial PO2 increased by a mean of 35 mm Hg (range, 4 to 110 mm Hg), permitting a decrease in inspired oxygen concentration or positive end-expiratory pressure when prone (4 patients); arterial PO2 decreased in 12 of 14 instances after the patient was turned from prone to supine. No significant change in mean arterial carbon dioxide tension, respiratory frequency, or effective compliance was observed.