The object of this study was to determine the clinically relevant dose of bendrofluazide for the treatment of arterial hypertension. A material of 257 male and female subjects, age 25-70 years, with sitting diastolic blood pressures between 100-120 mmHg after six weeks of placebo treatment participated in this randomized, double-blind placebo-controlled parallel group study. The patients were treated with either 0, 1.25, 2.5, 5.0 or 10 mg bendrofluazide daily for 12 weeks. Blood pressure was measured with a random zero sphygmomanometer and a Tricuff. Compliance was checked by tablet counts. The mean decreases in diastolic blood pressure were 3.5, 9.8, 10.8, 10.1 and 10.8 mmHg in the five treatment groups respectively. The heart rates were unchanged in all groups. Dose-effect relations were demonstrated for potassium, urate, glucose, cholesterol and apolipoprotein B. The lowest dose of bendrofluazide, 1.25 mg, affected only urate, whereas all of the mentioned biochemical variables were affected by the highest dose of 10 mg. It can be concluded that the optimal dose of bendrofluazide for aterial hypertension is 1.25 mg daily. Increase in the dose beyond this level only results in more pronounced adverse biochemical effects including the lipid-metabolism and subjective adverse events.