Tumor lysis syndrome (TLS) is a constellation of metabolic disturbances that may be observed in patients with malignancies. Clinically significant TLS can occur spontaneously, but most often is seen 48-72 h after initiation of cancer treatment. The metabolic abnormalities observed in patients with TLS include hyperkalemia, hyperuricemia, and hyperphosphatemia, which leads to secondary hypocalcemia. The precise incidence of TLS is not defined, risk factors being represented by large tumor burden, neoplasms with either high growth fraction or high sensitivity to chemotherapy, and by pre-existing impairment of renal function. Neither racial, nor sex predilection exists. The pathogenesis of TLS is related to the rapid tumor cell turnover or destruction, which may result in release of intracellular ions and metabolic byproducts into the systemic circulation. Acute renal failure (ARF) may frequently complicate TLS and is mainly due to renal tubule precipitation of uric acid, calcium phosphate, or hypoxanthine. Hemodynamic changes reducing glomerular flow due to still undefined mediators are also involved in TLS pathophysiology. Pre-existing volume depletion or renal dysfunction may worsen metabolic derangements and ARF. A good comprehension of TLS pathophysiology has provided the basis for an effective and rational treatment of this complication, adversely affecting the outcome of cancer patients.