Hyponatraemia is common, affecting about one in five of all hospitalized patients. Minor degrees of chronic hyponatraemia cause cognitive and motor impairment, and severe hyponatraemia is associated with substantial morbidity and mortality. Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common cause of hyponatraemia and is often poorly understood and inappropriately treated. Clinical evaluation and simple biochemical assessment should guide management. The introduction of vasopressin antagonists, or vaptans, into clinical practice heralds the beginning of a new and exciting era for this important group of disorders.