A new hypouricemic diuretic (tienilic acid) was compared with hydrochlorothiazide in a double-blind study in 8 patients with mild essential hypertension. After a two-week placebo period the patients received either 250 mg tienilic acid or 50 mg hydrochlorothiazide in a single daily dose for 3 weeks. After a second placebo period of 2 weeks the patients received, in a crossover design, either tienilic acid or hydrochlorothiazide for a further 3 weeks. The reduction of blood pressure and of body weight was similar for both drugs. When treatment was started diuresis and natriuresis increased with tienilic acid and with hydrochlorothiazide. Whereas serum sodium levels showed only minor variations, serum potassium levels fell with both diuretics and urinary potassium excretion increased. Urinary calcium excretion decreased and serum calcium levels slightly increased under both treatments. Both diuretics induced similar increases of plasma renin activity, plasma aldosterone concentration and aldosterone-18-glucuronide excretion. Blood urea nitrogen and, to a lesser extent, serum creatinine levels were raised slightly under both drug regimens. Whereas the serum uric acid level rose and remained elevated for the duration of hydrochlorothiazide treatment, it fell significantly and remained lowered during treatment with tienilic acid. Uric acid clearance was about twice as high with tienilic acid as with hydrochlorothiazide. Tienilic acid therefore appears to be a therapeutic alternative to thiazides and other hyperuricemic diuretics in hypertensive patients in whom hyperuricemia should be avoided or corrected.