Objective To study the protective effects of ulinastatin( UTI) on lung function after cardiopulmonary bypass( CPB) . Methods 42 Patients, ASA score Ⅱ ～Ⅲ, scheduled for elective cardiac valve replacement, were randomly allocated into three groups, ie. a control group, a low dose UTI group( UTI 8000U/kg) , and a high dose UTI group( UTI 12 000 U/kg) . Inspiratory pressure( PIP) , Plateau pressure ( Pplat) , alveolar-arterial oxygen pressure difference ( AaDO2 ) , static lung compliance ( Cs) and dynamic lung compliance ( Cd) were recorded before operation ( T1 ) and at 1 hour ( T2 ) , 4 hours ( T3 ) , 24 hours ( T4 ) after CPB termination. Results Compared with pre-CPB, postoperative PIP, Pplat and AaDO2 increased, and Cs and Cd decreased significantly in the control group( all P lt; 0. 05) . Compared with the control group at T2 ～T3 , postoperative PIP, Pplat, AaDO2 were significantly lower( P lt;0. 05) , and Cs and Cd were significantly higher in the two UTI groups( P lt;0. 05) . Compared with the low dose UTI group at T2 ～T3 , the PIP, Pplat and AaDO2 were significantly reduced( P lt;0. 05) , and the Cs and Cd were significantly increased in the high dose UTI group( P lt; 0. 05) . Conclusion UTI can alleviate lung injury and improve lung function during valve replacement surgery with CPB in a dose dependent manner.