Thiazide diuretics are widely used in the drug treatment of hypertension but their dose-response curves for the antihypertensive and adverse metabolic effects differ. To characterize the lower end of the dose-response curve a double-blind, parallel group trial was performed as multicentre study in Scandinavia. One hundred and eleven patients with newly diagnosed or previously treated mild to moderate hypertension (untreated diastolic blood pressure of 95-115 mmHg after 4 weeks placebo) were randomly allocated to various doses of hydrochlorothiazide (3, 6, 12.5 or 25 mg) or placebo for 6 weeks. Blood pressure and biochemical variables (plasma renin activity, serum potassium, magnesium, urate, fasting glucose, total cholesterol, HDL-cholesterol, triglycerides and apolipoproteins A1 and B were measured. 12.5 mg hydrochlorothiazide had a borderline effect on blood pressure whilst 25 mg had a definite antihypertensive effect. Biochemical changes were seen in plasma renin activity, serum potassium and urate after the 12.5 and 25 mg dose. Three and 6 mg had no effect on blood pressure or metabolic parameters.