TLS is a frequent metabolic complication of hematologic malignant diseases which can generate important renal impairment. As described in previous studies (Goldman S.C. et al, Blood 2001; 97, 2998–3003 and Pui C.H. et al., J.Clin.Oncol. 2001; 19, 697–704), there are High Risk (HR) patients (pts) of TLS. Rasburicase a recombinant urate oxidase enzyme transforms uric acid into the highly soluble compound allantoin which is then excreted by the kidneys. Rasburicase is highly effective in prevention and treatment of TLS. Based on these studies, the SFCE made the following recommandations for the management of TLS in children. HR pts are defined as : B cell Acute Lymphoblastic Leukemia (ALL), ALL or Acute Myeloblastic Leukemia (AML) with initial leukocyte count of at least 50x109/L, Stage III and IV T or B Non Hodgkin Lymphoma (NHL), any leukemia or NHL with a plasma uric acid concentration of at least 300 mmoles/L if <10 years old or 350 mmoles/L if >10 years old, serum creatinine or lactate deshydrogenase concentration (LDH) exciding twice the upper limit of normal (N), hyperphosphatemia ≥2mmoles/L. These pts are treated by hyperhydratation (3L/m2) ±alkaline and Rasburicase 0,20 mg/kg/d x 5days. Rasburicase is carried on if serum creatinine > 1,5N or phosphatemia >3mmoles/L or plasma uric acid concentration ≥200mmoles/L and until normalization. Low Risk (LR) pts are defined as pts not HR. They are treated by hyperhydratation (3L/m2) and Rasburicase 0,20 mg/kg/d for one day only. Rasburicase is carried on during 4 days or more on the same biological basis as HR pts. In order to check the validity of those guidelines, we are conducting a survey looking at the evolution of the pts managed according to the SFCE recommandations.150 pts are planned for a 6 months study. On July 30th, 39 pts are registred : 25 ALL, 6 AML, 8 NHL (3 Burkitt). At that time there is no SLT complication. Results will be presented at the 2004 ASH Meeting.