Inclusion Criteria:  Age of 18 years or more admitted to ICU. ARDS patients meeting the Berlin criteria under invasive mechanical ventilation within the first 24h. Mechanically ventilated patients with standard settings: PEEP of 5 cmH2O or more and VT 6 ml/kg PBW and Pplat equal to or below 28-30 cmH2O after intubation. PaO2/FIO2 between 150 and 300 mmHg under previous settings within 12 hours after mechanical ventilation standardization. Written informed consent obtained from the patient, next of kin/proxy or emergency consent. The patient will be asked to give his consent for the continuation of the trial when his condition will allow.  Exclusion Criteria:  Contra-indication to PP: spine instability; intracranial pressure greater than 20 mmHg; severe facial trauma; hemodynamic instability Non-inclusion criteria related to medical condition: Lung transplant; Burns over more than 20% of body surface; Chronic respiratory failure requiring home oxygen supplementation and/or non-invasive ventilation; Underlying disease with death expectancy within one year; Other non-inclusion criteria: Therapeutic limitation; inclusion into another interventional study in intubated and ventilated patients with mortality as primary end-point in the last 30 days; previously included in the same study; proning before inclusion. Individuals referred to in Articles L. 1121-5 à L. 1121-7 (pregnant women, persons deprived of freedom, minors), adult individuals under protection of law (L. 1121-8 and L. 1122-1-2) (emergency situations) of public health regulation in France Patients not affiliated to social security insurance regimen