Hyponatremia is a commonly encountered electrolyte disorder among hospitalized patients, and it is often underrecognized and undertreated. The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) is one of the causes of hyponatremia. Diagnosed by exclusion, failure to recognize it early may delay treatment and increase morbidity and mortality. The purpose of this article is to review the pathophysiology, causes, clinical features, diagnostic criteria, and current recommended treatments of SIADH. A case study of a hospitalized patient diagnosed with SIADH is presented.