To assess the diagnostic and prognostic value of renal venous renin levels in renovascular hypertension, 95 patients with severe unilateral renal artery stenosis were studied. Surgery (n = 52) or percutaneous transluminal dilation (n = 43) were done irrespective of renal venous renin levels. Lateralization of renin secretion as assessed by the PRA ratio and the renin secretion index was found in the majority of patients (66% and 88%, respectively). Patients with fibromuscular hyperplasia had more frequently PRA ratios