The effect of vitamin C on blood pressure is not well established. This is a randomized, double-blind control trial. Eligible patients were followed for 8 months. Patients were randomized to 500, 1000, or 2000 mg vitamin C. During each visit, a history including medication change was obtained and standardized blood pressure measurements were performed. A 1-week dietary diary was filled out before each visit. Multiple regression analysis and subsequent multiple comparisons were used for data analysis. Fifty-four patients satisfied our criteria and agreed to participate. Thirty-one patients (mean age, 62 +/- 2 years; 52% men, 90% whites) were randomized to the three doses of vitamin C. Overall compliance was 48 +/- 2%. Both mean systolic blood pressure (SBP) and diastolic blood pressure (DBP) decreased during the vitamin C supplementation phase [mean SBP dropped by 4.5 +/- 1.8 mm Hg (P <.05) and DBP by 2.8 +/- 1.2 mm Hg (P <.05)]. There was no difference between the three vitamin C groups (P =.48). This effect was significant for only 1 month of supplementation, but the trend persisted. There was no reported intolerance to vitamin C. There was no change in lipid levels after 6 months of treatment. Vitamin C supplementation lowers blood pressure in mildly hypertensive patients. There is no additional benefit for a higher dose than 500 mg daily. The effect of vitamin C is most likely to be only short term.