Fifty children with malignant diseases were vaccinated against hepatitis B. Twenty-nine children suffered from leukaemia or non-Hodgkin's lymphoma; 14 of these were on intensive chemotherapy (group I) and 15 were without intensive therapy (group II). The other 21 children had various forms of solid tumours, 14 of them were on intensive therapy (group III) and 7 were without intensive therapy (group IV). To evaluate the immune response, we determined antibody titres over a period of more than 14 weeks after the first vaccination. As 22 out of 50 patients had received passive immunisation together with either the first or the first and second vaccination, antibody titres at the 14th and 18th week (i.e. more than 10 weeks after passive immunisation) were used to evaluate the vaccination results. An antibody titre of -> 10 mIU/ml was considered to be a positive response. All patients of group IV, but only 4 out of 14 in group III, 4 out of 15 in group II, and 0 out of 14 in group I produced antibody titres higher than 50 mIU/ml. In contrast to the full response in group IV, twothirds of all other patients had no immune response (< 10 mIU/ml). Based on our experience we recommend vaccinating patients suffering from solid tumours and receiving no intensive therapy (group IV) against hepatitis B and protecting all the other children with malignant diseases by passive immunisation, if necessary.