In a double-blind study 30 patients with mild or moderate (World Health Organization classification I or II) hypertension were treated with either nifedipine, 20 mg twice a day, or bendroflumethiazide, 2.5 mg (+0.57 g KCl) twice a day for 24 weeks. All patients were then given a combination of half the initial dosage of both drugs for an additional period of 6 weeks. Nifedipine caused a significant reduction of both supine and standing blood pressures (16/10 and 11/5 mm Hg, respectively). Bendroflumethiazide also reduced both recumbent and standing blood pressures significantly (15/10 and 11/10 mm Hg, respectively). Combined treatment with nifedipine and bendroflumethiazide caused a further reduction of both supine and standing blood pressures. The total reduction compared with placebo was 23/12 mm Hg (p less than 0.001) and 17/11 mm Hg (p less than 0.001), respectively. There was no significant correlation between antihypertensive effect and age of patient.