A comparison of once-daily and twice-daily regimens of treatments with a beta-adrenoceptor-blocking agent, acebutolol, was carried out in patients with mild to moderate essential hypertension. Following an initial placebo phase, all patients entered an open 9-week period during which the dose of acebutolol, given on a twice-daily basis to all patients, was titrated to reduce seated diastolic blood pressure to less than 90 mm Hg or to decrease it by at least 10 mm Hg. After a further 4-week maintenance period, patients whose blood pressure had been successfully controlled entered a 3-month double-blind study in which they were randomly divided into three groups: those receiving continuation of twice-daily treatment (n = 47); those receiving once-daily treatment (using the same total daily dose as in the earlier twice-daily phase (n = 97); and those receiving placebo twice daily (n = 48). By the end of the study, 78% of the once-daily treatment group were still under control, a result similar to that of 72% in the twice-daily treatment group. In contrast, only 39% of the placebo patients remained under control. Moreover, the blood pressure in this group was significantly higher than in the other two groups. Acebutolol is an efficacious antihypertensive agent when used on a once-daily basis.