OBJECTIVE: To study the factors influencing mother-infant vertical transmission of hepatitis B virus (HBV). METHODS: A total of 635 pregnant women with chronic hepatitis B or chronic asymptomatic HBV carriers were enrolled. The rate of HBV infection was compared between the infants born from the pregnant women of different HBV-DNA load, different ways of delivery and different liver functions at birth and 3 months after birth. The newborn infants were routinely injected with hepatitis B immunoglobulin (200 IU) and hepatitis B vaccine (10 μg) within 12 hrs of birth. The newborns presenting HBV infection within 24 hrs of birth by serum test were re-injected with hepatitis B immunoglobulin (200 IU) 14 days after birth. RESULTS: The rate of HBV infection in infants with maternal HBV-DNA load >105 copies/mL was higher than in those with maternal HBV-DNA load ≤105copies/mL at birth (14.4% vs 4.1%; P<0.01) and 3 months after birth (4.7% vs 0; P<0.01).The rate of HBV infection at 3 months was lower than at birth in both groups. The rate of HBV infection in infants born by natural labor was higher than in those born by caesarean birth at birth (P<0.05), however, by 3 months after birth, the rate of HBV infection between the two groups was similar. The rate of HBV infection was higher in infants born to chronic asymptomatic HBV carrier mothers than that in infants born to chronic hepatitis B mothers at birth (P<0.01), but there were no significant differences in the two groups 3 months later. CONCLUSIONS: The maternal HBV-DNA load is correlated with the rate of HBV infection of infants. It might thus be an effective way to reduce the rate of HBV infection in infants by decreasing maternal HBV-DNA load. With the administration of hepatitis B immunoglobulin and hepatitis B vaccine, the delivery way and the liver function of pregnant women may not to be factors influencing mother-infant HBV vertical transmission.