The aim of this study was to correlate the seroconversion rate following hepatitis B vaccination in children with malignant diseases, to serum IG levels, the latter being an indirect method for determining B-cell function and humoral immunity status. One hundred and eight (108) patients (60 boys, 48 girls), 2-20 years old, with negative HBV, HCV and HIV markers, on or off chemotherapy, were studied. Of the 108 patients, 87 suffered from acute leukaemia, 11 from lymphoma, 7 from solid tumors and 3 had systematic Langerhan's cell histiocytosis. All patients were divided into 2 groups according to therapy status: group A comprised 70 patients under chemotherapy and group B 38 patients who had been off chemotherapy and in complete remission for 3 months to 10 years. Prior to the first vaccine dose, levels of IgG, IgA, IgM as well as IgG subclasses (G1, G2, G3, G4) were measured. Recombinant hepatitis B vaccine (20 mcg/dose) was administered at 0, 1, 2, and 6 months and the level of anti-HBs was determined one month after each dose. Protective titres were considered to be ≥ 10 mlU/ml. In group A the percentage of patients with normal values of IgG, IgA and IgM was 54%, 76% and 27% respectively, with a total seroconversion rate of 36%, whereas the corresponding percentage in group B were 97%, 100% and 71% with a total seroconversion rate of 82%. There was no significant correlation of immune response, either low or satisfactory, to any of the IgG subclasses. Our results support the hypothesis that the immunodeficiency status of patients under chemotherapy is responsible for the limited seroconversion rate to HBV vaccination. After the completion of therapy and the reconstitution of humoral immunity, a very good response to this vaccine is achieved.