The objective of this study was to investigate whether supplementation with soy protein, a rich source of oestrogen-like isoflavones, improves vascular function. Dutch investigators performed a 12-month double-blind RCT to compare the effects of soy protein containing 99 mg isoflavones a day (aglycone weights) with those of milk protein (placebo) on blood pressure and endothelial function in 202 post-menopausal women aged 60–75 years. Changes in endothelial function during the intervention were not significantly different between the soy and the placebo groups. After the intervention, systolic blood pressure increased in the soy group significantly more than it did in the placebo group; the difference in change was 4.3 mm Hg for systolic blood pressure but only 2.0 mm Hg for diastolic blood pressure. In the soy group only, systolic and diastolic blood pressure decreased and endothelial function improved in the equol producers, whereas systolic and diastolic blood pressure increased and endothelial function deteriorated in the equol non-producers.