Stephen MacMahon is a leading international authority on the risks associated with high blood pressure and the benefits resulting from blood pressure reduction. He was the first to demonstrate in epidemiological studies that there was no lower threshold, within the normal range of blood pressure, below which cardiovascular risks did not continue to decline. On this basis, he hypothesized that blood pressure reduction would benefit high-risk patients irrespective of their initial blood pressure level. He has since proven this in large-scale trials among patients with varying disease histories, the results of which have changed clinical guidelines and patient care worldwide.