Depletion of potassium is common in old people and is due to abnormal urinary excretion (misuse of diuretics, chronic pyelonephritis), to increased faecal elimination (misuse of laxatives, chronic diarrhoea) or to inadequate dietary intake. In a series of 90 elderly patients whose potassium status was investigated, the main manifestations of potassium deplition were weakness, increased sensitivity to digitials, impaired glucose tolerance and mental confusion. Potassium depletion can be most easily demonstrated by measurement of red-cell potassium levels; this method provides a valuable indication of the intracellular potassium content.