23 unselected patients with mild to moderate essential hypertension, whose average supine blood pressure after two months' observation on no treatment was 154/99 mm Hg, were entered into an eight week double blind randomised crossover study of one month's treatment with slow release potassium tablets (60 mmol/day) versus placebo without alteration of dietary sodium or potassium intake. By the fourth week mean supine blood pressure had fallen by 4% on potassium supplementation compared with placebo. Urinary potassium excretion increased from 62 +/- 4.7 mmol/24 h on placebo to 118 +/- 7.4 mmol/24 h on potassium. The fall in blood pressure was not related to urinary sodium excretion before entry to the trial or while on placebo. Moderate potassium supplementation caused a small but significant fall in blood pressure in patients with mild to moderate essential hypertension and could be additive to the effects of moderate sodium restriction. This increase in potassium intake could be achieved with a potassium-based salt substitute and a moderate increase in vegetable and fruit consumption. Moderate dietary sodium restriction with dietary potassium supplementation may obviate or reduce the need for drug treatment in some patients with mild to moderate hypertension.