The leading cause of hyponatremia in patients with cancer is syndrome of inappropriate antidiuretic hormone secretion (SIADH); this oncologic emergency requires immediate intervention. Left untreated, it can result in increased mortality and morbidity. A sodium level less than 135 meq/L is an electrolyte irregularity and defined as hyponatremia. It is extremely critical that oncology nurses are knowledgeable and able to evaluate and determine when patients are in fluid and electrolyte crisis. Nurses should be aware of the specific cancers and treatments that put patients at risk for developing hyponatremia. This article presents a case study for nursing consideration.