Tienilic acid (2,3-dichloro-4-(2-thienylcarbonyl) phenoxyacetic acid) is a new diuretic with uricosuric properties. Eighteen patients, aged between 37 and 67 years, with moderate arterial hypertension underwent a double-blind, within-patient, crossover study to compare the effects of tienilic acid (TNCF) and hydrochlorothiazide (HCTZ) on blood pressure, renal function, serum levels of uric acid and electrolytes, and liver function. Blood pressure was lowered similarly by TNCF and HCTZ. The prime advantage of TCNF over HCTZ was its profound hypouricaemic effect. Despite the possibility of hepatotoxicity of TNCF, it may still have a place in the treatment of hypertensive hyperuricaemic patients when the mechanism of hepatotoxicity of TNCF is elucidated.