K-rich fruit and vegetables may lower blood pressure (BP) and improve vascular function. A randomised controlled trial (ISRCTN50011192) with a cross-over design was conducted in free-living participants with early stages of hypertension (diastolic BP>80 and < 100 mmHg, not receiving BP-lowering medication) to test this hypothesis. Following a 3-week run-in period on a control diet, each subject completed four dietary 6-week dietary interventions (control+placebo capsules, an additional 20 or 40 mmol K+/d from fruit and vegetables or 40 mmol potassium citrate capsules/d) using a Latin square design with a washout period ≥ 5 weeks between the treatment periods. Out of fifty-seven subjects who were randomised, twenty-three male and twenty-five female participants completed the study; compliance to the intervention was corroborated by food intake records and increased urinary K+ excretion; plasma lipids, vitamin C, folate and homocysteine concentrations, urinary Na excretion, and body weight remained were unchanged. On the control diet, mean ambulatory 24 h systolic/diastolic BP were 132·3 (sd 12·0)/81·9 (sd 7·9) mmHg, and changes (Bonferroni's adjusted 95 % CI) compared with the control on the diets providing 20 and 40 mmol K+/d as fruit and vegetables were 0·8 ( − 3·5, 5·3)/0·8 ( − 1·9, 3·5) and 1·7 ( − 3·0, 5·3)/1·5 ( − 1·5, 4·4), respectively, and were 1·8 ( − 2·1, 5·8)/1·4 ( − 1·6, 4·4) mmHg on the 40 mmol potassium citrate supplement, and were not statistically significant. Arterial stiffness, endothelial function, and urinary and plasma isoprostane and C-reactive protein (CRP) concentrations did not differ significantly between the diets. The present study provides no evidence to support dietary advice to increase K intake above usual UK intakes in the subjects with early stages of hypertension.