More than 20 years ago, critical care workers first observed that oxygenation improved when patients with acute respiratory distress syndrome were ventilated in the prone position.In recent reports, on turning prone, from 50 to 100% of patients improve oxygenation to a degree sufficient to allow a reduction in the level of positive end-expiratory pressure or fraction of inspired oxygen. It appears that vascular conductance in lung regions previously in the dorsal position is augmented by an increase in air space volume, with the effect that prone position ventilation will reduce shunt and improve ventilation-perfusion mismatch. Factors determining which patients will respond have not yet been elucidated. Although many questions regarding the role of prone ventilation are unanswered, of greatest importance is whether this technique reduces morbidity and mortality of patients with acute respiratory failure. Only carefully conducted, randomized trials can answer this question.