BACKGROUND Fibromuscular dysplasia (FMD) and atherosclerotic renal artery stenosis (ARAS) are the most common causes of renovascular hypertension. So far, FMD is believed to cause hypertension via similar mechanisms as in ARAS, that is, a decrease in renal blood flow, which subsequently leads to increased renin secretion. However, given the differences in the blood pressure (BP)-lowering effect of revascularization between patients with ARAS and FMD, we questioned whether this is true. METHODS We measured renal blood flow (Xenon washout method) and renin secretion per kidney and their relationship to BP in a cohort of 64 patients with multifocal FMD and 110 patients with ARAS (off medication, prior to revascularization). RESULTS We found that renal blood flow is significantly higher in FMD as compared with ARAS. In patients with unilateral ARAS, renin secretion was increased in the affected kidney as compared with the unaffected kidney. This lateralization in renin secretion, however, was not found in unilateral FMD. After correction for differences in baseline characteristics, we found that systemic renin levels and local renin secretion was lower in FMD as compared with ARAS. Moreover, the relationship between BP and renin secretion in FMD was inverse to that in ARAS. CONCLUSION These findings argue against the hypothesis that FMD induces hypertension via similar pathophysiological mechanism as in ARAS.