Objective To investigate the concentration of 8-isoprostaglandin ( 8-iso-PG) in exhaled breath condensate ( EBC) of patients with acute lung injury/acute respiratory distress syndrome ( ALI/ ARDS) , and evaluate its clinical significance. Methods Thirty-one ALI/ARDS patients who received mechanical ventilation were recruited as an experimental group, and EBC was collected with improved EcoScreen condenser, while thirty normal subjects were recruited as a control group. The 8-iso-PG level in EBC was analyzed by ELISA. The ALI/ARDS patients were subdivided into a continuous ALI group ( n=10) , a continuous ARDS group ( n=9) , a worsening group ( n=6) , and a remission group ( n=6) by state of illness on 1st day and 5th day. The 8-iso-PG level in EBC of the ALI/ARDS patients with different severity and different prognosis were compared, and the correlation were analyzed between the 8-iso-PG level and clinical indicators. Results The 8-iso-PG levels in EBC of the ALI/ARDS patients on 1st day and 5th day were higher than those in the control subjects respectively [ ( 45.53 ±14.37) ng/L and ( 45.31 ±12.42) ng/L vs. ( 19.47 ±4.06) ng/L, Plt;0.001] . The 8-iso-PG level was higher in the worsening group than the remission group, higher in the continuous ARDS group than the continuous ALI group, and higher in the nonsurvival group than the survival group ( Plt;0.05) . The 8-iso-PG level in EBC was positively correlated with white blood cell count in peripheral blood, lung injury score, X-ray score, AaDO2 /PaO2 , AaDO2 , VT , VE and MEWS score, and negatively correlated with PaO2 /PAO2 and PaO2 /FiO2 . Conclusion The 8-iso-PG level in EBC of ALI/ARDS patients has reference value for judging severity and prognosis of ALI/ARDS.