BACKGROUND: Results of several studies published since 1999 suggest that primary hyperaldosteronism (also known as Conn's syndrome) affects more than 10% of people with hypertension; however, such a high prevalence has also been disputed. Experts generally agree that resistant hypertension has the highest prevalence of primary hyperaldosteronism, on the basis of small studies. We aimed to assess the prevalence of primary hyperaldosteronism in a large group of patients with resistant hypertension.METHODS: Patients with resistant hypertension (blood pressure >140/90 mm Hg despite a three drug regimen, including a diuretic) who attended our outpatient clinic were assessed for primary hyperaldosteronism. Serum aldosterone and plasma renin activity were determined and their ratio was calculated. Patients with a positive test (ratio >65.16 and aldosterone concentrations >416 pmol/L) underwent salt suppression tests with intravenous saline and fludrocortisone. Diagnosis of primary hyperaldosteronism was further confirmed by the response to treatment with spironolactone.FINDINGS: Over 20 years, we studied 1616 patients with resistant hypertension. 338 patients (20.9%) had a ratio of more than 65.16 and aldosterone concentrations of more than 416 pmol/L. On the basis of salt suppression tests, 182 (11.3%) patients had primary hyperaldosteronism, and response to spironolactone treatment further confirmed this diagnosis. Hypokalaemia was seen only in 83 patients with primary hyperaldosteronism (45.6%).INTERPRETATION: Although the prevalence of primary hyperaldosteronism in patients with resistant hypertension was high, it was substantially lower than previously reported. On the basis of this finding, we could assume that the prevalence of primary hyperaldosteronism in the general unselected hypertensive population is much lower than currently reported. Thus, the notion of an epidemic of primary hyperaldosteronism is not supported.