Hyponatremia is a common fluid and electrolyte disturbance in adults with cancer. Although a number of etiologies are associated with hyponatremia, the syndrome of inappropriate antidiuretic hormone (SIADH) secretion is one of the most common underlying causes. Early symptoms often associated with SIADH are subtle but, if left untreated, may progress to life-threatening seizures, coma, and death. Because oncology nurses have frequent and ongoing contact with patients, they are in an ideal position to recognize patients who are at increased risk for SIADH and those who present with early symptoms. Beginning signs and symptoms are mild and can be mistakenly attributed to other causes. This article reviews the pathophysiology of SIADH, associated risk factors, signs and symptoms, diagnosis, treatment, and nursing care.