Hyponatraemia is one of the most commonly encountered electrolyte abnormalities in general practice. Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is an important but under-recognised cause.This article explores the presentation, investigation, diagnosis and management of SIADH.SIADH can occur secondary to medications, malignancy, pulmonary disease, or any disorder involving the central nervous system. Diagnosis is made on the basis of clinical euvolaemic state with low serum sodium and osmolality, raised urine sodium and osmolality, and exclusion of pseudohyponatraemia and diuretic use. Fluid restriction of 800-1200 mL/24 hours is the mainstay of treatment. Patients with severe hyponatraemia and symptoms of altered mental state or seizures should be admitted to hospital for monitoring of fluid restriction and consideration of hypertonic saline. A rapid increase in sodium levels can precipitate osmotic demyelination and, as such, the increase in serum sodium should not exceed 10 mmol/L in 24 hours or 18 mmol/L in 48 hours.