Objectives To: (1) compare prevalence of diagnosed, measured, total and undiagnosed hypertension among late middle-aged adults in the United States of America and Ireland; (2) identify the most important predictors of hypertension and compare them across the two countries; (3) investigate whether cross-national differences in prevalence rates are explained by country differences in behavioural risk factors. Methods We use data from the 2008 and 2010 waves of the Health and Retirement Study (n = 1,938) and the first wave (2009/2011) of The Irish Longitudinal Study on Ageing (n = 1,455). We employ probit models to determine whether individual attributes, socioeconomic characteristics and behavioural health factors, including smoking, alcohol consumption and obesity, are associated with hypertension. We do not consider whether respondents are on antihypertensive medication. Results Prevalence of diagnosed hypertension is higher in the United States of America (48.6 versus 32.4 %). Prevalence of undiagnosed hypertension is higher in Ireland (41.2 versus 19.7 %). Little of the difference in prevalence rates is explained by country differences in behavioural risk factors. Conclusions A greater focus on prevention of high blood pressure is necessary in Ireland.