A study of hyponatraemia in the in-patients of a Geriatric Department during a 10-month period showed that 77 patients (11.3%) had plasma sodium concentrations below 130 mmol/1. Thirty-one patients (4.5%) had severe hyponatraemia of less than 125 mmol/l. Seventy-three per cent of hyponatraemias were iatrogenic caused by diuretic or intravenous fluid therapy. Hydrochlorothiazide/amiloride combination has the greatest tendency to produce hyponatraemia compared to all other diuretics ( P <0.01). Clinical features attributable to hyponatraemia were present in 61 %. Nine patients needed hospital admission solely due to hyponatraemia. The mortality rate for hyponatraemia was twice the overall rate for the unit.