Background. Children with acute lymphoblastic leukemia (ALL) carry a high risk of hepatitis B virus (HIV) infection. The present study was conducted to see if prior routine hepatitis B vaccine received as a part Of national immunization program could prevent HBV infection in these children. Methodology: Ninety-six children with ALL were screened for HBV Children were divided into three groups according to their initial HBV serology; previously vaccinated children (Group I) (n=34) previously unvaccinated and seronegative children (Group II) (n=56), and unvaccinated but HBsAg negative and anti-HBs positive children (group III) (n=6). Sixty-seven of 96 (69.7%) children received vaccination. The schedule was initiated during the third month of maintenance therapy and each course consisted of three doses given at one month interval, Results: Anti-HBs seroconversion following the first course of three doses of hepatitis B vaccination in group I, II and III was 57916, 33% and 100%, respectively. It increased to, 97% in Group I, 62.5% in Group II, 100% in Group III. HBsAg positivity was found in 11 children (H. 5516) and all of them developed chronic hepatitis B. Ten of them were in Group II whereas only one child was in Group I (P<0.04). Conclusion: This data reveals that routine HBV vaccination within the national immunization program plays an important role in decreasing subsequent hepatitis B infection in children with ALL.