There Has been considerable discussion about the accuracy and interpretation of the isotope renogram (1–8). Usually, the renogram has been evaluated on the basis of other examinations. Often arteriography has been considered the criterion, but it has been shown that arterial stenoses are not necessarily related to hypertension (3). The Howard test (9) and the excretory urogram are interpreted largely on the basis of comparison of the two sides. In bilateral abnormality these tests indicate only the more severely impaired kidney. This study was undertaken to evaluate the renogram by more definitive criteria, namely, the study of resected tissue and the one-year postoperative follow-up of patients undergoing definitive procedures. The renograms of 52 patients who meet obtained on the day prior to the examination in case contrast material is needed to identify the kidneys accurately. The patient is positioned as he will be for the reno-gram, prone with the head of the table raised 30 degrees. Patients maintain this position beneath the probes easily, and gravity promotes drainage. The probes should be placed over the geometrical centers of the kidneys. To avoid parallax each kidney is localized separately by film or fluoroscopy. To standardize the degree of dehydration, the patient is given no food or fluid after midnight prior to the examination, and the study itself is performed only between the hours of eight and twelve in the morning. With this moderate dehydration, differences between normal and abnormal kidneys are amplified, and criteria defining the presence or absence of abnormalities can more readily be set. Ideally, the patient is off all antihypertensive medications, especially those of the diuretic type, since they tend to prevent the excessive reabsorption of sodium and water by an ischemic kidney. Other types of antihypertensive medication also mask reno-graphic abnormalities. The renogram should precede any retrograde ureteral catheterization study and renal angiography since both of these pro; cedures can produce abnormal tracings or false positives. Thus, the renogram should be the first major diagnostic test performed in the study of the patient. One microcurie of iodine-131 orthoiodohippurate (Hippuran) I is administered for each 8 kg of body weight. If any of the material infiltrates the tissues around the vein, the descending limb of the tracing will be markedly prolonged and the test will be invalid. This should be checked in any case of doubt by placing a probe over the site of injection. 1 From the Department of Radiology (W. R. E.,Chairman; L. A. Du S., Physicist; E. J. K., former Resident) and the Division of Hypertension (J. R. C), The Henry Ford Hospital, Detroit, Mich. Accepted for publication in Tune 1967. 2 Present address: William Beaumont Hospital, Royal Oak, Mich. RADIOLOGY 89: 667–675, October 1967.