The efficacy of 3 schemes of passive and active prevention of HBV infection was evaluated in 47 children with haematologic proliferative diseases. Twenty-six children suffering from leukemia (group I) received passive immunisation (hepatitis B immunoglobulin) in six week intervals during intensive chemotherapy and were vaccinated on maintenance therapy. Thirteen children with Hodgkin or B-non-Hodgkin lymphoma (group II) received active immunisation from the beginning of intensive chemotherapy with two doses of immunoglobulin. Eight children who had completed their therapy (group III) were vaccinated only. Among children who completed vaccinations, 5/8 in group I, 4/7 in group II and 5/5 in group III produced protective anti-HBs levels. Passive/active prophylaxis was successful in most patients suffering from neoplastic diseases and reduced the endemy of HBV infection in our department from 43.3% to 2.56% infected subjects. Among 7 patients vaccinated from the beginning of treatment (group II), 4 of them produced protective levels of anti,-HBs, despite intensive chemotherapy.