Study Selection
We included studies that met the following criteria: adult patients with acute hypoxemic respiratory failure (Pao2 /Fio2 ≤ 300mm Hg), including acute lung injury (ALI) and ARDS, who were under mechanical ventilatory support; all studies that randomly assigned patients to two or more groups, including prone or supine positioning, during ventilation; and all-cause mortality was reported regardless of the timing of data collection. We excluded RCTs conducted on pediatric patients and randomized crossover trials that assigned patients to both prone and supine groups.