Since the introduction of the aldosterone-to-renin ratio (ARR ) as a screening tool for primary aldosteronism (PA), there has been a marked increase in the reported prevalence of this condition among hypertensive subjects. A meta-analysis from the literature shows a PA prevalence of almost 8% among hypertensive patients, with a twofold higher prevalence in referred patients as compared with primary care patients (9.0 vs 4.3%). However, the usefulness of the ARR remains subject of debate, because of doubts on its validity, and the many factors affecting the ARR , including posture, time of day of blood sampling, and use of antihypertensive medication. Furthermore, there is no clear cut-off value and it is unknown what population should be screened. Recently, The Dutch ARR AT Study was initiated. This is a multicentre, prospective trial aiming to evaluate the test characteristics of the ARR within a Dutch population of therapy-resistant hypertensive patients. The effect of antihypertensive medication on the ARR will be studied. Furthermore, from this study the prevalence of PA in this population will follow. Last, the blood pressure response to the selective aldosterone-receptor-antagonist eplerenone will be evaluated. The Dutch ARR AT Study will run until the end of 2009 and will contribute to the formulation of uniform guidelines for the screening for PA in the Netherlands.