Children with cancer are a risk group for hepatitis B virus (HBV) infection. In Poland, the initiation of a national HBV vaccination programme in neonates and infants in 1995 has contributed to the prevention of HBV infection in children treated for neoplastic diseases. The objective of this study was to analyse the anti-HBs pattern and occurrence of HBV infections in children with cancer who had been vaccinated during infancy. The study included 96 children divided into three groups: Group A, children who had received a full vaccination course with an initial level of anti-HBs >/=100 IU/L; Group B, children who had received a full vaccination course with anti-HBs <100 IU/L, for whom an additional dose of vaccine was administered; and Group C, children who had further immunoprophylaxis because they did not complete the vaccination course before cancer diagnosis. A protective level of anti-HBs after the full vaccination programme was found in 80.5% of children after three months, 74.2% after six months, 61.5% after 12 months and 78.6% after 18 months. Among children who produced antibodies, a slow decrease in the level of anti-HBs was observed. It was still protective during follow-up in Group A, but occasionally fell below 100 IU/L in Group B. In Group C, five of 11 subjects had a protective level of anti-HBs throughout follow-up. Of 28 children who showed the presence of HBsAg during follow-up, 23 eradicated the virus. In children with cancer vaccinated against HBV according to the vaccination schedule, the immune response maintains a protective level of anti-HBs in more than 60% of cases, despite immunosuppression.