Abstract: Objective To investigate the influence of cardiopul monary bypass(CPB) to the cellular immune function of T lymphocyte. Me th ods Among 500 patients operated from March 2006 to September 2006，30 patients with rheumatic heart disease were selected randomly as the CPB group, which would replace mitral valve; 30 patients with congenital patent ductus arte reriosus as the nonCPB group, which would ligate ductus arteriosus without CPB . The blood was sampled before operation, at the end of CPB or operation, and 24 hours after operation. After T lymphocyte was seperated, the quantum o f T lymphocyte, apoptosis of T lymphocyte, ability of T lymphocyte to kill tumou r cell were measured. Results The quantum of T lymphocyte i n CPB group at the end of CPB was decreased than that before operation （50.9% ±6.8% vs. 58.5%± 9.1%,Plt;0.05）; apoptosis of T lymphocyte at the end of CPB and 24 hou rs after operation were increased than that before operation （6.5%±2.2% vs. 0. 9%±1.1%, 5.6%±1.8% vs. 0.9%±1.1%;Plt;0.01）; ability to kill tumour cell b reakdown in CPB group at the end of CPB and 24 hours after operation was decrea sed than that before operation （30.4%±6.0% vs. 37.3%±8.6%, 29.0%±4.9% vs . 37 .3%±8.6%;Plt;0.05）. Ability to kill tumour cell breakdown in CPB group was lower than that in nonCPB group at the end of CPB （30.4%±6.0% vs. 33.6%±5. 3%, Plt;0.05）. Conclusion CPB can depress the cellular im mune function,which causes temporary immune depression to the body.