Objective To explore the potential mechanism of breakthrough bleeding associated with progestin with in vitro methods.Methods The isolation and culture of human endometrial endothelial cells（HEECs）was performed with the method established in our laboratory.The content and activity of urokinase-type plasminogen activator（uPA）and the content of plasminogen activator inhibitor-1（PAI-1）in cell supernatants after incubated with different concentrations of progesterone（0-5 μmol/L）and 17β-estradiol（0

0.1

or 1 nmol/L）were measured by method of ELISA.Apoptosis rate of HEECs was measured by flow cytometry.Viable cell count was measured by MTT.Results The increased level of progesterone（0.5-5 μmol/L）combined with 17β-estradiol elevated content and activity of uPA while the production of PAI-1 remained unchanged.The apoptosis of HEECs was inhibited along with the increment of total viable cell counts at higher concentrations of progesterone with 17β-estradiol.Conclusion The inhibition of apoptosis and increased content and activity of uPA may contribute to the occurrence of irregular bleeding associated with progestin use to some extent.