Twelve patients with essential hypertension who were already on treatment with the long-acting calcium antagonist amlodipine (5 mg once daily) were entered into a double-blind, randomized crossover study of the addition of one month's treatment with either bendrofluazide (5 mg once daily) or matching placebo. The addition of bendrofluazide did not cause any statistically significant fall in the supine blood pressure compared to treatment with placebo (147.6/90.1 +/- 4.8/2.8 v 150.8/92.6 +/- 4.3/2.3 mm Hg, respectively). Plasma potassium was significantly lower on bendrofluazide as compared to placebo (3.11 +/- 0.14 v 3.62v +/- 0.13 mmol/L, P less than .001) and 10 of 12 patients had a fall in plasma potassium while on diuretic. The results of this study suggest that a thiazide diuretic has little additive effect on blood pressure in patients already on the long-acting dihydropyridine amlodipine, and may cause hypokalemia.