The correlation between the arteriographic appearance and functional significance of renal artery stenosis in hypertensive patients is poor. Measurements of renal vein renin in patients with unilateral renal artery stenosis undergoing renal angioplasty show that before angioplasty there is lateralization of renin secretion to the ischemic kidney, with suppression from the contralateral kidney. Following angioplasty this pattern returns to normal, with both kidneys showing similar increments of renin in the renal veins. The prediction of therapeutic outcome from renal vein renin measurements is more reliable (sensitivity 74%, specificity 100%) when expressed as the increment of renin for each renal vein over the inferior vena caval value than as the ratio between the two renal veins (sensitivity 62%, specificity 60%), but there is a high false-negative rate with both methods. Renal vein renin patterns cannot reliably distinguish between unilateral and bilateral renal artery stenosis, and in patients with bilateral stenoses tend to show lateralization to the more ischemic kidney.