Background. The purpose of this study was to evaluate the impact of hepatitis B prophylaxis in preventing hepatitis B infection in children with malignancy. Procedure, Between May, 1993, and September, 1998, a total of 151 children (95 boys, 56 girls), 29 (19%) with lymphoma, 58 (39%) with leukemia, and 64 (42%) with solid tumor, were screened for hepatitis B Virus (HBV). The mean age was 7.5 +/- 2.5 years. Children with negative serology; received active and/or passive immunization. HBsAg and anti-HBs were positive prior to vaccination in 16 (10%) and 17 (11%) children, respectively. One hundred eighteen children (78%) of one hundred fifty-one with negative serology were included in the vaccination program. The vaccine dose was 40 mu g. Children With solid tumor and lymphoma received recombinant hepatitis B vaccine at diagnosis, repeated at months 1, 2, and 12. Hyperimmunglobulin was administered monthly in children with leukemia during the intensive chemotherapy period. They were then vaccinated following the third month of maintenance therapy with the schedule described above. Anti-HBs titers were measured every 3 months, and titers above 10 mlU/ml were accepted as protective. Results. Anti-HBs positivity after the first three doses was 77% in solid tumors, 88% in acute leukemia, and 48% in lymphomas. Anti-HBs positivity with respect to diagnosis in children completing the vaccination schedule was 94% in solid tumor, 90% in leukemia, and 74% in lymphoma (P > 0.05). Thirty-three percent of children have not received the fourth dose as yet. In total 78% of the children developed protective antibody titers with or without the fourth dose, and none was infected with HBV during 3 years of follow-up. Ten (39%) of twenty-six children who remained unresponsive to immunization were infected with HBV. Conclusions. These data reveal that HBV prophylaxis is necessary and that our vaccination schedule is effective in preventing HBV infection in these children. (C) 2000 Wiley-Liss, Inc.