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In acute respiratory failure, classified as acute lung injury (ALI) or the more severe acute respiratory distress syndrome (ARDS), prone position ventilation (PPV) can improve oxygenation by recruiting alveoli situated in dorsal-dependent regions of the lung and by alteration of the ventilation/perfusion ratio. The aim of our study is to analyze the prognostic value of the change of oxygenation after starting prone position ventilation in ARF.