 Current definitions of the Acute Respiratory Distress Syndrome ARDS could be adequate for epidemiological studies but are not adequate for inclusion of patients into therapeutic clinical trials. It is a matter of debate whether the assessment of hypoxemia at ARDS onset is appropriate for stratifying lung severity and risk of death in ARDS patients. The investigators will perform an observational, non-interventional, multicenter, prospective audit in a network of intensive care units in Spain for evaluating the severity and risk of death based on the assessment of respiratory and ventilatory function at 24 hours after ARDS diagnosis under standardized ventilatory conditions. This study is the confirmatory phase of the study NCT02288949. 