In the time period from February to October 1988 the indicators of hepatitis virus B infection, transaminase activity, and bilirubin level were studied in 89 children with acute leukaemias and malignant lymphomas. The age of the children was from 7 months to 16 years. Fifteen children were examined before starting the treatment with cytostatics, 48 during intensive chemotherapy, and 26 during maintenance treatment. HBV markers were found in 53 children during intensive therapy and maintenance treatment. Out of 36 children with negative HBV infection markers 31 were examined before starting the treatment with cytostatics or during the first 6 months of treatment. Nine out of 53 children in whom HBV infection markers were found had aminotransferase activity raised over 100 u/l and/or bilirubin level over 5 mg/dl and in nine cases transaminase activity and bilirubin level were raised only slightly (below 100 u/l and below 5 wg/dl). In 53 children the results were normal. The high incidence of HBV infection in children with acute leukaemias and lymphomas indicates the necessity of routine prophylactic treatment in this group of patients.