Objective:To determine the levels of of MPO expression in human serum and placenta during pregnancy and their correlation with the development of preeclampsia. Methods:Patients with mild preeclampsia(n=33)and severe preeclampsia(n=30)were recruited while thirty-one women with normal pregnancies served as controls.Serum samples from uterine venous blood and peripheral venous blood and placentas were collected. Serum concentrations of MPO were determined with a chromatography method whereas the expression of MPO in placentas was detected by immunohistochemistry. Results:① Serum levels of MPO in uterine venous blood from both mild and severe preeclampsia groups were significantly higher than those of healthy controls(P < 0.01),with the highest levels detected in severe preeclampsia group(P < 0.01). ② Although serum levels of MPO in the peripheral venous blood from both mild and severe preeclampsia groups were higher than those of healthy pregnant group,there were no significant differences among three groups. ③ The levels of MPO expression in placentas of preeclampsia groups were significantly higher than those of healthy pregnant women,with the highest levels detected in severe preeclampsia patients(P < 0.01;P=0.017). Conclusion:The increased levels of MPO in uterine venous blood and placentas are closely correlated with the development of preeclampsia,suggesting that MPO may play a role in the pathogenesis of preeclampsia via local oxidative stress.