Abstract： Objective To assess the effect of prone position on the oxygenation of patients with acute respiratory distress syndrome (ARDS) after recruitment maneuvers (RM) failure at high altitudes.Methods Forty-one ARDS patients who were invalid for RM therapy at the altitude of 2260 metres area hospital were enrolled [ mean oxygenation index (PaO2/FiO2) increased than RM＜ 20％ considered as RM invalid ].All were divided into acute respiratory distress syndrome (ARDSp) and extra-acute respiratory distress syndrome (ARDSexp) based on different etiology,and each group was randomly divided into the supine and the prone position group,i.e.that ARDSp prone position group (n=11),ARDSp supine group (n=9),ARDSexp prone position group (n=10),and ARDSexp supine group (n =11 ).Before ventilation and 1,2,3,4 hours after ventilation,arterial partial pressure of oxygen (PaO2),PaO2/FiO2,static lung compliance (Cst),airway resistance (Raw) were monitored.Results One hour after ventilation in ARDSexp prone position group,PaO2/FiO2 (mm Hg,1 mm Hg =0.133 kPa ) was significantly higher than before ventilation (157.4 ± 40.6 vs.129.3 ±48.7,P＜0.05),and increased by ventilation time,peaked at 4 hours (219.1 ±41.1 ).Within 3 hours ventilation in ARDSexp prone position group,PaO2/FiO2 was significantly higher compared with the other three groups,and there were no significant differences among the other three groups.PaO2/FiO2 in ARDSp prone position group and ARDSexp prone position group at 4 hours of ventilation were significantly higher compared with the corresponding supine group (208.8 ± 39.7 vs.127.4 ± 47.1,219.1 ± 41.1 vs.124.9 ± 50.8,both P＜0.05 ).There was no statistically significant difference in Cst before and after ventilation,and there was no significant changes among the groups.Raw (cm H2O ·L-1 ·s-1) in ARDSp prone position group at 4 hours of ventilation were significantly reduced than before ventilation (6.8 ± 1.7 vs.10.7 ± 1.8,P＜0.05 ),and significantly lower than the other three groups.There was no statistically significant difference in RAW among other three groups at each time point.Conclusion Prone position can improve RM oxygenation invalided ARDS patients at high altitudes.