BACKGROUND A double-blind randomized controlled trial was carried out to assess the effect of sour milk, containing two tripeptides (valine-proline-proline and isoleucine-proline-proline), on blood pressure (BP). METHODS A total of 46 borderline hypertensive men aged 23 to 59 years were recruited at their workplace for this trial. Subjects were randomly allocated into two groups; sour milk drink group (S-group, n = 23) and placebo (acidified milk) drink group (P-group, n = 23) for 4 weeks. Blood pressure was measured twice at each occasion by a physician, at the health center of the company, with a mercury at baseline, 2 and 4 weeks. Statistical analysis was performed by SPSS 10.0J. RESULTS The S-group and P-group showed no significant difference in baseline systolic BP (mean [SD], S: 147.6 [9.6], P: 145.3 [13.0]) or diastolic BP (S: 95.3 [9.9], P: 91.5 [9.6]). In the S-group, change in systolic BP at 2 and 4 weeks were -4.3 mm Hg (95% confidence interval [CI] -8.3 to -0.4; P = .032) and -5.2 mm Hg (95% CI -10.1 to -0.3; P = .039), both statistically significant. Diastolic BP showed change from -1.7 mm Hg (95% CI -5.4 to 2.0) at 2 weeks and -2.0 (95% CI -5.4 to 1.5) at 4 weeks, respectively. In the P-group, change in systolic BP were -0.5 (95% CI -5.8 to 4.8) at 2 weeks and -3.7 (95% CI -8.3 to 0.9) and change in diastolic BP were -0.6 (95% CI -4.7 to 3.6) and -0.3 (95% CI -3.9 to 3.3), which were not statistically significant. CONCLUSIONS This trial demonstrated the beneficial effect of sour milk on BP in borderline hypertensive men who were not taking antihypertensive medication.