Renovascular disease due to progressive atherosclerotic renal artery stenosis is being diagnosed with increasing frequency in the elderly. At what degree of renal artery stenosis should intervention be recommended is not clear. To answer this question, unilateral or bilateral activation of the renin-angiotensin system or its absence were detected by captopril-stimulated renal vein renin measurements in 49 hypertensive patients, aged 63 years, with normal or near-normal renal function (serum creatinine concentration < or =2.0 mg/dL), and the information was matched against radiographic measurements of the extent of renal artery stenosis. With few exceptions, unilateral or bilateral hypersecretion of renin was associated with 80% or greater reduction of renal artery lumen diameter. In contrast, normal secretion or suppression of renin production in a kidney contralateral to an ischemic one was associated with either normal caliber renal artery or renal artery stenosis less than 80%. These findings suggest that renal artery stenosis less than 80% should be monitored rather than treated because improvement of renal function and amelioration of hypertension are not expected unless the renin-angiotensin system has been activated in the affected kidney. Renoprotection by early intervention is uncertain because progression of renal artery stenosis is unpredictable. Normal captopril-stimulated renal vein renin measurements in hypertensive patients obviate the need for further work-up or interventional therapy of renovascular disease.