One hundred and two newly diagnosed children with malignancy followed up at the Pediatric Oncology Department of Ankara Oncology Hospital were investigated for hepatitis B, hepatitis C and human immunodeficiency virus markers. The ages ranged between seven months and 17 years with a median of 10 years; 62 were male. Twenty-four patients had Hodgkin's disease, 11 had non-Hodgkin's lymphoma and 67 patients had other solid tumours. Four patients had positive HBs-Ag, 15 developed immunity against hepatitis B and had anti-HBs antibodies. One patient had a previous infection with positive anti-HBc. Hepatitis C virus antibodies were positive in only one patient. HIV serology was negative in all patients. These results show the high prevalence of hepatitis B virus infection in paediatric oncology patients. In countries where hepatitis B is endemic, strict screening of blood donors, usage of disposable equipment and vaccination of patients is recommended. Although the incidence of hepatitis C and human immunodeficiency virus infections are rising, infection with hepatitis B is still by far the most common.