The validity of noninvasive (iodine-131 iodohippurate renogram, iodine-131 ortho-iodohippurate clearance, indium-113m EDTA--technetium-99m DTPA sequential renal scan) and invasive (xenon-133 washout) radionuclide screening tests was evaluated in the diagnosis of 105 patients with unilateral renovascular hypertension (RVH) and in 45 patients with essential hypertension (EH). In RVH positive findings on the stenosed side were noted in 73% of renograms, 73% of o-iodohippurate-clearance tests (N = 22), 81% of sequential renal scans, and 90% of xenon-washout studies (N = 67). In a subgroup of 55 retrospectively selected patients with normal or improved blood pressure following renovascular surgery, the preoperative findings had been positive on the stenosed side in 78% of renograms, 75% of o-iodohippurate-clearance tests (n = 20), 85% of sequential renal scans, and 93% of xenon-washout studies (n = 23). The sequential renal scan appears to be a sufficiently reliable method in noninvasive screening for unilateral RVH, although invasive xenon-washout studies show a higher percentage of hemodynamic alterations in the stenosed kidney. o-iodohippurate clearance tests, and in particular xenon-washout studies, can reveal arteriosclerotic lesions in the contralateral, non-stenosed kidney, which may be of importance when the decision for renovascular surgery is pending.