Objective To study endothelial damage by observing changes of circulating endothelial cells (CECs) in blood, coagulation and fibrinolysis index in patients with acute respiratory distress syndrome.Methods CECs were separated by isopycnic centrifugation method in 14 patients with acute lung injury (ALI), 7patients with acute respiratory distress syndrome (ARDS), 10 intensive care unit (ICU) controls, and 15 healthy controls.Plasma prothrombin time (PT), activated partial thromboplastin time (APTT), fibrinogen (FG), fibrin degradation products (FDP), and D-dimer were examined simultaneously. Acute physiology and chronic health evaluation (APACHE) Ⅱ and lung injury score (LIS) were recorded to evaluate severity of illness and lung injury.Results (1) The number of CECs in ALI (10.4 ± 2.3 ) and ARDS groups ( 16.1 ± 2.7) was higher than that in the healthy (1.9 ± 0.5) (P ＜ 0.01). In both ALI and ARDS, the number of CECs correlated with APACHE Ⅱ (r = 0.55, P ＜ 0.05 and r =0.62, P ＜ 0.05, respectively) and LIS (r = 0.60, P ＜ 0.05 and r = 0.53, P ＜ 0.05, respectively). CEC number was negatively correlated with PaO2 in ALI and ARDS (r=-0.49, P＜ 0.05 and r=-0.64, P＜ 0.05, respectively). (2) The level of FDP and D-dirmer were higher in ALI and ARDS patients than that in ICU and healthy control groups (P＜0.05). The level of FG in ARDS group was significantly higher than in the ICU and healthy control groups (P ＜ 0.05). But in ALI group, the level of FG was significantly higher than only healthy control group (P ＜ 0.05).Conclusions Endothelial cell damage occurs in ARDS patients, which may play a major role in the pathophysiology of ARDS. Changes of endothelial cell activation and damage markers, such as CECs, plasma coagulation and fibrinolysis index,to some extent reflect severity of illness and lung injury in ARDS.