Since the study is prospective, with consecutive collection of clinically relevant data from patients and with the participation of several ICUs from different geographical regions of Spain, the findings would be highly generalizable. The study is aimed to improve the knowledge on the epidemiology and outcome of ventilated patients, to identify the percentage of patients developing ARDS under the current definition, to validate a bedside scoring system developed by us for stratification of lung severity, and to examine the prognosis of hypoxemic respiratory failure, and risk factors associated with fatal outcome.