An unexpected result of our work concerns the change over time of respiratory mechanics, blood gas analysis and VD alv . For many years, our PP protocol has been to turn patients to PP for up to 15 to 18 hours per day for 3 days [15] . In the study by Mancebo et al. [30] , which concluded that PP may reduce mortality in patients with severe ARDS, PP sessions lasted 20 hours/ day. In a recent study, we demonstrated that PP sessions that lasted 18 hours/day were independently associated with survival [31] . In the present study, the maximum effect of PP for VD alv , PaCO 2 and Pplat occurred six to nine hours after turning patients to PP. Later the effect seemed to be a decline. How this affects the effect of PP on patient prognosis remains to be elucidated.