Paper AbstractThe diagnosis of primary hyperaldosteronism due to microadenoma or unilateral adrenal hyperplasia can be challenging, since hypokalaemic alkalosis, high plasma aldosterone and a definite adenoma on imaging may all be absent. Method and result. We describe three cases of resistant hypertension (on > 5 antihypertensives) where hyperaldosteronism was suspected because of a suppressed plasma renin level despite treatment with multiple drugs which normally elevate renin. Renin mass was measured by a double-site chemi-immunoluminometric assay.All patients had normal plasma aldosterone levels. Hypokalaemia was present in the first two cases but computed tomography did not show clear cut adenomas.Adrenal vein sampling (AVS) revealed lateralisation (> 4 times higher aldosterone to cortisol ratio (ACR) on the affected than contra-lateral side).The third patient was normokalaemic and AVS showed only minimal lateralisation (ACR 1.3:1).The severe hypertension in all cases was reversed by adrenalectomy, with blood pressure falling to target despite withdrawal of all but one to two drugs. Conclusions. The robotic assay of renin mass permits rapid detection of patients in whom plasma renin is suppressed below the normal range.A suppressed plasma renin indicates abnormal Na + -retention, and -when not overcome by drugs such as angiotensinconverting enzyme-inhibitors or angiotensin receptor blockers -may be the only clue to a curable adrenal adenoma.AVS is required to demonstrate lateralisation of aldosterone secretion, justifying adrenalectomy.