In 25 children with lymphoblastic leukemia is shown that prophylactic treatment with allopurinol, 48 hours before initial cytostatic therapy, always prevents hyperuricemia. In 4 patients was renounced on this measurement and 2 of 4 patients died of the sequeles of hyperuricemia. In severe ill leukemic patients the parental use of allopurinol is recommended. 2 additional patients developed during remission therapy or after cessation of chemotherapy a moderate hyperuricemia which had to be treated, over years, with allopurinol.