SummaryTwenty-two patients with mild to moderate hypertension participated in a double-blind trial during 7 weeks in order to compare the antihypertensive effect and biochemical properties of tienilic acid 250 nig u.i.d. and hydrochlorothiazide 50 mg u.i.d. Reduction of systolic and diastolic blood pressure was similar with both drugs. At therapeutic doses, a slight decrease in scrum potassium was observed in both groups. Tienilic acid significantly decreased whereas hydrochlorothiazide significantly increased serum urate levels. Both diuretics were well tolerated. This study confirms that, in contrast to other thiazide diuretics, tienilic acid produces an important hypo-uricaemic effect.