The antihypertensive effect and patient tolerability during 12 weeks' treatment with atenolol and bendroflumethiazide were evaluated in an open, randomized, between-patient trial. Out of a total of 162 patients, aged 50-75 years, with previously untreated hypertension, 151 completed the trial. They were randomly allocated to two groups. Forty-nine patients, aged 50-64 years (middle-aged), and 23, aged 65-75 years (elderly), were treated with atenolol. Forty middle-aged and 39 elderly were treated with bendroflumethiazide. Significant reductions in blood pressure (BP) were observed during treatment with each drug (p less than 0.001). The change in diastolic BP in middle-aged patients was significant in favour of atenolol (p less than 0.01), but otherwise no difference was found between the two drugs. Uric acid increased during treatment with both drugs (p less than 0.001). Serum potassium decreased during bendroflumethiazide treatment (p less than 0.001). Subjective side-effects of both drugs were few and expected. The results of this study indicate that atenolol and bendroflumethiazide are equally effective in reducing BP in patients aged 50-75 years.