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The diagnosis of curable, secondary causes of hypertension is a satisfying but challenging aspect, for both patient and doctor, of the clinical management of hypertension. They are probably under-recognized, and the enthusiasm to look for causes is blunted by low pickup rates in unselected patients. Young age encourages a search more because the alternative is many decades of drug treatment than because of any good evidence that prevalence of secondary hypertension varies with age.1 Plasma renin is still not a routine measurement even in the younger patient. And although there is an increasing evidence that a low plasma renin is valuable in the detection of primary aldosteronism,2 a high plasma renin does not usually trigger investigations for renal artery stenosis. The cases we present here are not part of a formal prospective series. But they illustrate how valuable was the finding of an extremely high plasma renin, leading to re-evaluation of magnetic resonance (MR) and computed tomographic (CT) angiograms in the light of a high, rather than low prior probability. Our report is stimulated by the coincidence of 5 such patients being diagnosed within an 18-month period, and a parallel prospective study of plasma renin concentration’s prediction of treatment response, which enabled us to define the threshold for an extremely high plasma renin.3 We will discuss the significance of the log distribution of renin, of factors such as drug treatment influencing interpretation, and of reviewing reportedly normal radiology. Plasma renin concentration was measured by the Diasorin Liaison automated immunoassay analyzer.4 Intra-assay coefficients of variation were 3.7%, 2.8%, 2.0%, and 1.2% at concentrations of 15.1, 33.8, 82.2, and 258.0 mU/L, respectively. Inter-assay coefficients of variation were 5.8% and 4.9% at concentrations of 25.6 and 101.3 mU/L, respectively. The lower limit of quantitation of the assay, as specified …