Aim To investigate the effect of oxidized low density lipoprotein(ox-LDL)on proliferation,apoptosis,and expression of bcl-2 of endothelial progenitor cells(EPC)from human umbilical cord blood in vitro.Methods Total mononuclear cells were isolated from human umbilical cord blood in vitro by Ficoll density gradient centrifugation and then the cells were plated on fibronectin-coated culture dishes.After 7 d of culture,attached cells were stimulated with different cencentrations of ox-LDL(5,10 and 20 mg/L)for 48 h.EPC were identified by demonstrating the expression of CD34,KDR and CD133 under a laser scanning confocal microscope.MTT assay was used to detect the effect of ox-LDL on the multiplication ability of EPC.Flow cytometry was used to detect the apoptosis caused by ox-LDL.The expression of bcl-2 genes mRNA and protein were detected respectively by RT-PCR and immunohistochemistry technology.Results After exposure to ox-LDL,the proliferation of EPC was lower than that of control group,apoptosis rate was higher than that of control group(p<0.01),and it was dose-dependent in experiment range;expression of bcl-2 mRNA and protein were down-regulated(p<0.05).Conclusion Ox-LDL can inhibit the proliferation of EPC and promote the apoptosis of the cells by down-regulation of bcl-2 expression,which may contribute to vasculogenesis and reparation of blood vessel endothelium.