Rasburicase Versus Allopurinol in Tumor Patients at Risk for Hyperuricemia and Tumor Lysis Syndrome

Inclusion Criteria: 1. Meets high risk or at potential risk for tumor lysis syndrome (TLS): A patient is at high risk for TLS if he/she presents with: * Hyperuricemia of malignancy (plasma uric acid \> 7.5 mg/dL); * A diagnosis of very aggressive lymphoma/leukemia based on the Revised European-American Lymphoma (REAL) classification of lymphoma/leukemia; * Acute myeloid leukemia (AML); * Chronic myeloid leukemia (CML) in blast crisis; or * High grade myelodysplastic syndrome (refractory anemia with excess blast, chronic myelomonocytic leukemia, and refractory anemia with excess blast in transformation) only if they have \> 10% bone marrow blast involvement and are given aggressive treatment similar to AML. A patient is at potential risk for TLS if he/she presents with: * A diagnosis of aggressive lymphoma/leukemia based on the REAL classification of lymphoma/leukemia plus 1 or more of the following criteria: * Lactate dehydrogenase (LDH) \>= 2 x upper limit of normal (ULN) (IU/L) * Stage III-IV disease * Stage I-II disease with at least 1 lymph node/tumor \> 5 cm in diameter In addition to the above-mentioned eligibility criteria, the patients should have the following criteria: 2. Eastern Cooperative Oncology Group (ECOG) performance status 0-3 3. Age \>= 18 years 4. Life expectancy \> 3 months 5. Negative pregnancy test (females of child bearing potential) and use of effective contraceptive method (for both males and females). A pregnancy test may be performed on serum or urine human chorionic gonadotropin (HCG). 6. Signed written informed consent