A double-blind trial was carried out in three parallel groups of patients with mild to moderate hypertension to assess the efficacy and tolerance of piretanide given as a single dose of 3 mg, 6 mg or 12 mg per day over a period of 12 weeks. Active drug treatment in the 90 patients studied was preceded and followed by 2-week periods on placebo. The results showed that although all three groups had significant reductions in systolic and diastolic blood pressures, both at rest and standing, compared with initial levels, the decrease was significantly greater in the two higher dosage groups. Pulse and respiratory rates also decreased and body weight was reduced slightly. The biochemical and haematological parameters in all three groups showed no clinically relevant changes during treatment with piretanide. Minor side-effects definitely or probably associated with piretanide treatment were observed in all three groups but were generally mild and did not interfere with treatment. Only 1 of the 9 patients who withdrew prematurely from the trial did so because of lack of effect, 1 patient withdrew because of side-effects and the other 6 patients who dropped out for drug-related reasons did so because the antihypertensive effects were too marked at the 6 mg (3 patients) or 12 mg (3 patients) per day dosage level.