HYPOTHESIS AND OBJECTIVES: The investigators hypothesize that adjunctive treatment with intravenous dexamethasone of patients with established ARDS might change the pulmonary and systemic inflammatory response and thereby will increase the number of ventilator-free days and will decrease the extremely high overall mortality. Our goal is to examine the effects of dexamethasone on length of duration of mechanical ventilation (assessed by number of ventilator-free days) and on mortality, in patients admitted into a network of Spanish intensive care units (ICUs) who still meet ARDS criteria at 24 hours after ARDS onset.