The syndrome of inappropriate antidiuretic hormone (SIADH) is reported as the most common cause of hyponatraemia. This retrospective cross-sectional study evaluated the diagnosis of SIADH in 110 hospitalised patients in an Australian tertiary hospital with reference to recently published clinical guidelines. Investigation of SIADH was incomplete in all but 20% of cases. Adrenal insufficiency and hypothyroidism were not excluded in a significant number of cases.