A double-blind study was carried out in two parallel groups of patients with mild to moderate hypertension to assess the efficacy and tolerance of the combination 20 mg penbutolol plus 3 mg piretanide in comparison to 40 mg penbutolol alone over a period of 6 weeks. Active drug treatment in the 51 patients studied was preceded by a 2-week period of placebo. The results showed that in both groups there was an effective reduction in systolic and diastolic blood pressure compared with initial levels. Although there was no significant difference between the groups, the normalization of diastolic blood pressure (less than 95 mmHg) was achieved in 70% of the patients receiving the combination and in 59% of the patients treated with penbutolol alone. Pulse rate decreased in both groups, body weight only in the combination group. The biochemical and haematological parameters showed no clinically relevant changes during treatment with either drug regimens. Minor side-effects definitely or probably associated with the treatment were observed in both groups but were generally mild and did not interfere with treatment. No patient withdrew prematurely from the trial.