To the Editor.— The article by Thibonnier et al1is seriously flawed by several problems. The first is the matter of renal artery stenosis causing hypertension. This is a retrospective diagnosis that is realized only when correction of the stenosis or removal of the kidney results in a sustained cure of the hypertension. The authors present no evidence to suggest a sustained cure of elevated blood pressure in their patients (average follow-up of three months). Therefore, what they appear to have shown is that 100% of patients with unilateral renal artery stenosis will have a renal vein renin ratio of 1.5 or more after captopril therapy. Surely they do not believe that every patient with unilateral renal artery stenosis has renovascular hypertension, as seems to be their claim. If this is true, then one would not need to measure renal vein renins at all but only look at the