Acute tumor lysis syndrome and associated renal failure are frequently seen in children with B cell leukemia and lymphoma. In patients resistant to conservative management, renal replacement therapy is required. Continuous arteriovenous hémodiafiltration (CAVHD) and continuous arteriovenous hemofiltration (CAVH) were used successfully in the treatment of tumor lysis syndrome and renal failure preceding chemotherapy in a child with B cell leukemia. The advantages of the methods are discussed.