Tumor lysis syndrome (TLS) describes the clinical and laboratory sequelae that result from the rapid release of intracellular contents of dying cancer cells. It is characterized by the release of potassium, phosphorous, and nucleic acids from cancer cells into the blood stream, with the potential to cause hyperkalemia; hyperphosphatemia and secondary hypocalcemia; hyperuricemia; AKI; and, should usual homeostatic mechanisms fail, death. TLS most commonly follows treatment of hematologic malignancies, such as acute lymphocytic or lymphoblastic leukemia, acute myeloid leukemia, and Burkitt lymphoma, but also occurs after treatment of other bulky or rapidly growing tumors, particularly if the patient is highly sensitive to the effects of cytotoxic chemotherapy. Prevention and treatment depend on prompt recognition of patients at risk, volume repletion, allopurinol, rasburicase (a novel recombinant urate oxidase), and, when indicated, dialysis.