Tumor lysis syndrome occurs when there is excessive rapid cell death with release of intracellular contents. This massive release of cellular contents results in hyperkalemia, hyperphosphatemia, hypocalcemia, hyperuricemia, acute renal insufficiency/failure, cardiac arrythmias, or seizures. The history is important as most patients have recently started chemotherapy and then present with oliguria, seizures, or arrythmias prompting further work-up with a laboratory investigation.  The renal impairment responds to IV fluid resuscitation which is the main form of therapy. Severe hyperkalemia should be treated appropriately. Dietary phosphate restriction is usually sufficient to treat the hyperphosphatemia and hypocalemia. Xanthine oxidase inhibitors are usually given in anticipation of tumor lysis syndrome in appropriate patients undergoing chemotherapy.