1. A total of 450 hypertensive patients above the age of 60 years have entered the double-blind multicentre trial of the European Working Party on High blood pressure in Elderly (EWPHE). After stratification and randomization half were treated with one capsule daily containing 25 mg of hydrochlorothiazide and 50 mg of triamterene and half were given placebo. In those receiving active treatment, if blood pressure control was not adequate they were given a second capsule and if necessary up to 2 g of methyldopa/day. 2. No significant differences between the groups were present before randomization. A significant blood pressure difference of 25/10 mmHg was obtained between the groups and maintained during 4 years of follow-up. No major disturbances in serum potassium or serum sodium were noted with the present drug combination. 3. During the initial phase an increase in serum creatinine and serum uric acid was noted in the actively treated group, which was maintained during the later years. This increase in serum creatinine in the actively treated group was related (P = 0·003 and r = −0·247) to the decrease in sitting systolic blood pressure. Changes in serum uric acid were (r = 0·3 and P = 0·003) correlated with the changes in serum creatinine both in the placebo and in the actively treated group, but independent of the change in creatinine; the serum uric acid was on average 1 mg higher in the actively treated than in the placebo group. 4. Fasting blood glucose did not change significantly in the placebo-treated group but in the active treatment group the rise was statistically significant. 5. A favourable influence on prognosis by active treatment can be expected on the basis of the blood pressure reduction and in the absence of major electrolytes disturbances. However, the balance between this decreased risk and the increased risk produced by the rise in blood glucose and the other treatment effects remains to be determined. Therefore the trial continues and more patients are being admitted.