Objective: To explore the role of polymorphonuclear neutrophils (PMN) activation in lung injury following cardiopulmonary bypass (CPB). Methods: Twenty-four cases of patients with rheumatic heart disease were enrolled in this study to detect the situation of PMN activation and the degree of lung injury. Bronchoalveolar lavage fluid (BALF) and blood samples were collected just after anaesthesia and 4 h postoperatively. Related clinic data were recorded. Results: This cohort included 4 male and 20 female with age ranging from 29 to 69 years old, and body mass from 37 to 73 kg. Mean cardiopulmonary bypass time was (106.46±33.58) min, mean cross clamp time was (77.58±28.02) min, and mean mechanical ventilation time was (24.17± 30.90) h. Postoperative PaO2/FiO2 decreased significantly than that during preoperation (P=0. 000). The postoperative WBC counts in peripheral blood and in BALF were both increased significantly than those during preoperation. And the postoperative rates of PMN in BALF were also increased significantly. Postoperative neutriphil elastase (NE) and myeloperoxidase (MPO) released from inflammatory cells were increased significantly (P=0. 000) both in plasma and BALF. The postoperative von Willebrand factor (vWF) concentration in plasma was also increased significantly (P=0. 000).The postoperative concentration of sLPI in plasma was decreased,whereas it was increased significantly in BALF. Furthermore, there was a negative correlation between NE and PaO2/FiO2 both in BALF and in plasma. A negative correlation was present between vWF in plasma and PaO2/FiO2. A positive correlation was found between sLPI in plasma and PaO2/FiO2. Conclusion: PMN is recruited and activated after CPB and released large quantity of proteases, and lead to lung injury. sLPI is an important protective factor against inflammatory injury.