Two patients with Burkitt's lymphoma and acute renal failure are described, one with acute uric acid nephropathy and the other with acute renal failure due to hyperphosphataemia. Renal insufficiency caused by the precipitation of calcium phosphate salts only occurs after starting treatment of the lymphoma; uric acid nephropathy can also be present in an untreated patient. A uric acid/creatinine ratio in a random urine sample may help to confirm the diagnosis of acute uric acid nephropathy. Vigorous hydration, allopurinol and alkalinization of the urine have been advocated to prevent uric acid nephropathy. However, alkalinization may accelerate phosphate precipitation in the kidneys and thereby induce renal failure. Xanthine nephropathy may develop as a consequence of high doses of allopurinol. The administration of large volumes of fluid therefore remains the keystone in prevention of the tumour lysis syndrome.