Randomized controlled pilot trial in 70 patients with Acute Respiratory Distress Syndrome (ARDS) ventilated with low tidal volumes and limitation on airway pressure at 35 centimeters of water (cmH2O), to compare two different methods of selecting the level of Positive End-Expiratory Pressure (PEEP) to be applied: according to fraction of inspired oxygen (FiO2) needed or individualized according to the best compliance. Primary objective was evolution of arterial oxygenation during the 28 days. Secondary objectives were to measure its effects on hemodynamic parameters, 28-day mortality, number of ventilator-free days at day 28, Intensive Care Unit (ICU) and hospital stay, number of multiple-organ dysfunction-free days and a multivariate analysis of 28 day-mortality.