Renal artery stenosis is a narrowing of the artery that supplies blood to the kidney, a condition that can cause high blood pressure and eventually lead to kidney failure. The most accurate way to test for renal artery stenosis is to x-ray the renal arteries after injecting them directly with dye (intra-arterial angiography), which is invasive and sometimes dangerous. Several other noninvasive, less dangerous tests can help diagnose renal artery stenosis by showing the anatomy of arteries, detecting changes in blood flow, or measuring kidney function. These include computed tomography angiography (CTA), magnetic resonance angiography (MRA), ultrasonography (US), captopril renal scintigraphy (CRS), and the simple captopril test. CTA uses x-ray beams, and MRA uses radio waves and a magnetic field to take pictures of the renal arteries. For CTA, dye is injected, while MRA can be performed with or without dye. US detects abnormal blood flow in arteries by using sound waves. For CRS and the captopril test, patients are given captopril, a drug that affects blood flow to the kidneys. The CRS scans show how well a radioactive substance injected into a vein is picked up in the kidney after captopril is given. The simple captopril test uses a blood sample to measure a substance (renin) released by the kidneys when blood flow to the kidney is low. It is not known which of the above tests is the most accurate in detecting renal artery stenosis.