Background. Hepatitis B is an important public health concern in South Africa (SA). The hepatitis B virus (HBV) vaccine was introduced into the South African Expanded Programme on Immunisation (EPI-SA) in 1995. There is no 'catch-up' programme in place. The duration of protection after hepatitis B vaccination in the SA population is unknown. Waning of vaccine-induced immunity leaves people at risk of acquiring hepatitis B infection in settings where the prevalence of infection is high and horizontal transmission is likely.Objective. To assess immunity to HBV in patients at presentation to a paediatric haematology and oncology unit.Methods. An audit of hepatitis profiles was done of all new patients seen in the unit from January 2012 to December 2013. Patients were classified as immune (antibody levels to hepatitis B surface antigen (anti-HBs) >100 mIU/ml), low immune (anti-HBs 10 - 100 mIU/ml) and not immune (anti-HBs <10 mIU/ml).Results. Of the 210 patients included (median age 6.5 years), 84 (40.0%) had no immunity to hepatitis B despite presumed vaccination as part of the EPI schedule. Six patients tested positive for hepatitis B core antibody (anti-HBc), consistent with previous infection. No patients had active hepatitis B infection (hepatitis B surface antigen-positive). Most human immunodeficiency virus (HIV)-infected patients were not immune to HBV (80.0%).Conclusion. A significant number of children in SA are not immune to hepatitis B despite vaccination being part of the EPI-SA. Combined passive-active immunisation should be considered for all oncology patients in settings where exposure to HBV is possible. Consideration should also be given to offering booster vaccination to the population as a whole.