An elderly woman who had been taking a fixed-dose combination of a thiazide and potassium-sparing diuretic for eight days, became severely hyponatraemic following total hip replacement. Her hyponatraemia resolved with fluid restriction, intravenous normal saline, and withdrawal of the drug. Hyponatraemia is a well-described side-effect of diuretic therapy; however, we are not aware of any previous reports of this condition developing acutely in the postoperative state. We attribute this to the natriuretic nature of the drug, compounded by the postoperative, anti-diuretic surge which itself may have been exacerbated by the drug. We thus advise caution in the use of this preparation in the elderly patient about to undergo major surgery.