Abstract: Objective To investigate the expression and significance of CD4⁺ CD25⁺ regulatory T cells in peripheral blood from newborns with hepatitis B virus(HBV) intrauterine infection.Methods We selected 79 hepatitis B virus surface antigen( HBsAg) positive pregant women and their newborns.The HBV markers of the pregnant women and the neonates were detected with enzyme-linked immunosorbant assay(ELISA).HBV DNA contents of pregnant women and neonates were detected with real-time fluorescent quantitative-PCR(RT-PCR).The expression of CD4⁺ CD25⁺ regulatory T cells(Tregs) in peripheral blood from the newborns were detected and analyzed with flow cytometer.Results The percentage of CD4⁺CD25⁺Tregs/CD4⁺ and CD4⁺CD25⁺Foxp3⁺ Tregs/CD4⁺ were 10.57?2.25% and 1.67?0.37% in peripheral blood from the newborns of intrauterine infection group and were significantly higher than those of non-intrauterine infection group(P<0.05).The expression of CD4⁺CD25⁺ Tregs and CD4⁺CD25⁺Foxp3⁺ Tregs increased with maternal blood HBV DNA level.The proportion of CD4⁺CD25⁺ Tregs was significantly higher than that of negative group(P<0.05).The expression of CD4⁺CD25⁺ Tregs and CD4⁺CD25⁺Foxp3⁺ Tregs were correlated positively with HBV DNA level in pregnant mothers(r=0.430,r=0.409,P<0.05).Conclusion CD4⁺CD25⁺ regulatory T cells could inhibit the elimination of HBV through suppressing cellular immune response,and its upregulation of expression may be responsible for HBV inntrauterine infection.