Abstract Peripheral and renal vein renin activity was measured in 46 hypertensive patients with arteriographically diagnosed unilateral renal artery stenosis before and six months after technically successful renal angioplasty. The renin-sodium profile was high in 80 percent before angioplasty, fell in all patients, and became normal or low in 85 percent after angioplasty. Renal vein renin activity, which initially showed lateralization of renin secretion to the ischemic kidney with contralateral suppression, became normal. Renal vein renin activity was more reliable for predicting the therapeutic response when expressed as the increment of renin for each renal vein over the infrarenal vena caval value (sensitivity 74 percent, specificity 100 percent) than as the ratio between the two renal veins (sensitivity 62 percent, specificity 60 percent). The predictive value of renal vein renin activity is poor when plasma renin activity is stimulated by long-term administration of captopril. These data support the usefulness and define the limitations of peripheral and renal vein renin measurements in selecting patients for treatment by renal angioplasty.