Elevated blood pressure (BP), or hypertension, is a growing burden worldwide, leading to over 10 million deaths each year. May Measurement Month (MMM) is a global initiative aimed at raising awareness of high BP and acting as a stimulus to improving screening programmes worldwide. In the United Kingdom (UK) nearly 1 in 5 people, and in the Republic of Ireland (RoI) 3 out of 10, have hypertension, of which a large proportion remains undiagnosed. An opportunistic cross-sectional survey of volunteers aged ≥18 years was carried out in May 2017. Blood pressure measurement, the definition of hypertension and statistical analysis followed a standardized protocol. Screenings sites in hospitals, universities, shopping centres, workplaces, sports clubs, community centres, GP practices, and pharmacies were set up across the UK and RoI as part of this initiative. Seven thousand seven hundred and fourteen individuals were screened during MMM17. After multiple imputation, 3099 (40.3%) had hypertension. Of individuals not receiving antihypertensive medication, 1406 (23.4%) were hypertensive. Of individuals receiving antihypertensive medication, 682 (40.5%) had uncontrolled BP. MMM17 was the largest BP screening campaign ever undertaken in the UK and RoI. These data prove for the first time that a relatively inexpensive, volunteer based, convenience sampling of screening BP in the community identified two out of five individuals as hypertensive, with one in four not receiving treatment. Of major concern is that these data demonstrate that of those individuals receiving treatment, two out of five still did not have controlled BP.