 The American-European Consensus Conference AECC and the Berlin definitions of the Acute Respiratory Distress Syndrome ARDS could be adequate for epidemiologic studies, but it is not adequate for inclusion of patients into therapeutic clinical trials. Despite recent reports on the effects of standardized ventilator settings on PaO2FIO2 and fulfillment of AECC and Berlin definitions of ARDS, it is still a matter of debate whether the assessment of hypoxemia at 24 hours is the most appropriate tool for stratifying lung severity in patients with ARDS. The investigators will perform an observational, multicenter, prospective audit in a network of intensive care units in Spain and China for validating and confirming that the assessment of hypoxemia at 24 hours after ARDS onset is the most valuable tool for stratifying and predicting outcome in patients with ARDS. 