In our unit, patients with a PaO 2 /FiO 2 ratio < 100 mmHg after 24 to 48 hours of mechanical ventilation are systematically turned to PP when hemodynamically stable [15] . Our study was approved by the Ethics Committee of the "Société de Réanimation de Langue Française" (SRLF-CE 07-213). After obtaining informed consent from the patients' relatives, 15 patients were included in the study between January 2008 and March 2010. Inclusion criteria were (1) the presence of ARDS according to the definition of the Acute Respiratory Distress Syndrome Network [3] ; (2) persistence of severe hypoxemia after 48 hours of mechanical ventilation, defined as a PaO 2 /FiO 2 ratio < 100 mmHg; and (3) hemodynamic stability, defined as systolic blood pressure > 90 mmHg with norepinephrine infusion at a rate < 0.5 μg/kg/minute. Patients with chronic obstructive pulmonary disease were excluded.