Mechanical ventilation is typically done with the patient in the semiupright position. However, in patients with ARDS, prone positioning may result in better oxygenation primarily by creating more uniform ventilation. Uniform ventilation reduces the amount of lung that has no ventilation (ie, the amount of shunt), which is generally greatest in the dorsal and caudal lung regions, while having minimal effects on perfusion distribution.Although many investigators advocate a trial of prone positioning in patients with ARDS who require high levels of PEEP (eg, > 12 cm H2O) and Fio2 (eg, > 0.6), until recently trials have not shown any improvement in mortality with this strategy (however, these trials have typically been underpowered). A recent, large, multicenter, prospective trial assessed patients who had severe ARDS (PaO2:FIO2< 150 mm Hg on an FIO2 �� 0.6, PEEP > 5 cm H2O) and who were on a tidal volume of about 6 mL/kg. These patients were randomized to undergo �� 16 h of prone positioning or be left in the supine position during ventilation. The study, which included a total of 466 patients, identified lower 28- and 90-day mortality in the prone-positioning group without a significant incidence of associated complications. Prone positioning is contraindicated in patients with spinal instability or increased intracranial pressure. This position also requires careful attention by the ICU staff to avoid complications, such as dislodgement of the endotracheal tube or intravascular catheters.