The accuracy and reliability of quantitative gamma camera renography in screening for functionally significant renovascular hypertension was investigated. Renal perfusion, total and relative renal function, and parenchymal transit times were measured in 141 patients with recently diagnosed or severe hypertension. Renal angiography was performed in 13 of the 15 patients diagnosed as having evidence of renovascular hypertension. In all 13 either occlusion or significant stenosis of the renal artery was correctly predicted. Previous studies utilizing static images, playback buffers, and renograms have shown that these methods have excellent sensitivity, but relatively low specificity because of false-positive results. The addition of computer quantitation, including measurement of parenchymal transit times, has greatly improved the accuracy of this test. Quantitative renography can now be regarded as a reliable means of selecting patients for transluminal dilatation of stenosed renal arteries and for follow-up after this procedure.