119 elderly, hypertensive patients were followed-up for 1 year and 48 for 2 years in a double-blind, randomised, controlled trial in which they received either placebo or 25-50 mg hydrochlorothiazide and 50-100 mg of triamterene daily. Half of the active treatment group also received 250 mg to 2 g methyldopa daily. After 2 years the active treatment group had an average increase in fasting blood-sugar of 9.6 mg/dl compared with an average fall of 3.1 mg in the placebo group (p less than 0.001). Blood-glucose rose by an average of 26.6 mg/dl in the active group when determined 1 hour after 50 g oral glucose and decreased by an average of 5.3 mg/dl in patients who had been on placebo for two years (p less than 0.05). The hyperglycaemic effect of diuretics appeared to be related to potassium loss since, in both groups, impairment of glucose tolerance was most marked in those in whom the serum-potassium decreased over the 2 years.