Standard prophylaxis and treatment of malignancy-associated hyperuricemia in the USA has been allopurinol with vigorous hydration, urinary alkalinization and osmotic diuresis. Urate oxidase, the enzyme that converts uric acid to allantoin (a readily excreted metabolite that has 5- to 10-fold higher solubility than uric acid), is an alternative therapy; however, few published findings support this practice. Between February 1994 and December 1996, we administered non-recombinant urate oxidase (Uricozyme) to 126 children with newly diagnosed non-B cell acute lymphoblastic leukemia (ALL) during the first 5 days of chemotherapy with methotrexate, 6-mercaptopurine or both. Their blood levels of uric acid and other indicators of tumor lysis were measured at diagnosis and during treatment and then compared with findings in 129 similarly treated historical controls who had received allopurinol to control hyperuricemia. Clinical responses to urate oxidase were also determined in eight patients with newly diagnosed B cell ALL or advanced-stage non-Hodgkin lymphoma. Patients treated with urate oxidase had rapid and significantly greater decreases in their blood uric acid levels than did the historical controls (median maximal level during treatment, 2.3 vs 3.9 mg/dl, P < 0.001). They also had lower creatinine (0.6 vs 0.7 mg/dl, P = 0.01) and blood urea nitrogen (11 vs 24 mg/dl, P < 0.001) levels. Similar findings were made in the eight cases of B cell ALL or non-Hodgkin lymphoma. None of the patients required dialysis for acute renal failure. Six (4.5%) of the 134 children given urate oxidase had allergic reactions, manifested primarily by urticaria, bronchospasm and hypoxemia. Thus, non-recombinant urate oxidase is a more effective uricolytic agent than allopurinol but is associated with acute hypersensitivity reactions, even in patients without a history of allergy.