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To determine the effect of oral vitamin C supplements on ambulatory blood pressure and plasma lipids.A 6-month double-blind randomized placebo-controlled cross-over study with a 1-week washout between cross-over periods.Vitamin C 500 mg daily or matching placebo was given to 40 men and women aged between 60 and 80 years for 3 months each in a cross-over fashion. Clinic and 24-h ambulatory blood pressure, plasma ascorbate and lipids were measured at baseline and at the end of each cross-over phase.Clinic blood pressure did not change between placebo and vitamin C phases. Daytime ambulatory blood pressure showed a small but significant fall in systolic blood pressure (2.0 ± 5.2 mmHg; 95% confidence interval 0–3.9 mmHg) but not in diastolic blood pressure. Regression analysis showed that with increasing baseline daytime blood pressure the fall in blood pressure with vitamin C supplementation increased. Regression analysis of the change in high-density lipoprotein (HDL) cholesterol showed a significant effect of sex on the change in HDL cholesterol. In women, but not men, HDL cholesterol increased significantly by 0.08 ± 0.11 mmol/l, P = 0.007. There was no change in low-density lipoprotein cholesterol between treatment periods.In older adults high intakes of ascorbic acid have modest effects on lowering high systolic blood pressure, which could contribute to the reported association between higher vitamin C intake and lower risk of cardiovascular disease and stroke.