Acute renal failure during treatment of lymphoblastic malignancies is usually due to drug toxicity or acute uric nephropathy. Observations were recently reported where extreme hyperphosphatemia may represent another pathophysiological mechanism. We describe 2 cases, in 36- and 77-year-old women, with acute lymphoblastic leukemia. Acute renal failure was observed 2 days after cytotoxic treatment. Maximal blood creatinine values were 860 and 550 mumol/l respectively, and for phosphate 6.3 and 7.5 mmol/l. With oral phosphate binders, and after four peritoneal exchanges for the second patient, renal function gradually returned to normal values within 4 weeks. Tumor lysis syndrome with associated hyperphosphatemia is described exclusively in lymphoblastic malignancies and renal failure is probably a consequence of intratubular calcium phosphate precipitation. In this situation prophylactic administration of phosphate binders and attentive monitoring of phosphatemia are necessary.