The character of a renal mass may now be identified with reasonable exactness by means of translumbar aortography and nephrotomography. Nephrotomography is a combination of rapid intravenous nephrography and body-section radiography (tomography) and, in our opinion, is simpler and less hazardous than aortography. The essential features of the examination as initially used were (a) an intravenous aortogram, (b) a plain nephrogram, and (c) a nephrotomogram. The plain nephrogram proved to have no specific value and therefore has been eliminated. The addition of certain other minor modifications in technic perhaps justifies a recapitulation of the technical aspects of the examination. 1. The patient is positioned on a radiographic table equipped for tomography. Prior to the examination the entire procedure is carefully explained and rehearsed. 2. Preliminary test tomograms are obtained to determine the level which best visualizes the kidneys and to check technic. 3. Following this, a No. 12 gauge Robb-Steinberg angiocardiographic needle is inserted into an antecubital vein. 4. Decholin circulation-time determinations are made to permit an estimate of the time of exposure of the aortogram phase and to familiarize the patient with the taste of Decholin. 5. Twenty-five cubic centimeters of 50 per cent Hypaque or Miokon is then injected, for the purpose of re-enforcing the later nephrogram as well as for visualizing the collecting system at the time of injection of the concentrated medium. 6. A 50 c.c. syringe is filled with one of the concentrated organic iodinated contrast agents to which has been added 5 c.c, of Decholin. We have used 70 per cent Urokon and more recently 90 per cent Hypaque. Preliminary experience with the latter has revealed it to be a highly satisfactory medium, extremely well tolerated. The injection is made as rapidly as possible, according to angiocardiographic technic, the time not exceeding 2.0 seconds. The first exposure is made at the Decholin end-point and is designed to capture the opacified aorta and renal arteries. 7. Immediately following this, the first tomogram is taken at the preselected level. Additional tomograms are then taken at 1 cm. above and 1 cm. below that level. 8. All films are then reviewed. If the findings are not conclusive, we have not hesitated to do a second injection. Results An analysis of the first 272 consecutive nephrotomographic studies revealed that twenty were so inadequate technically as not to permit a diagnosis. In the remaining 252 cases a specific diagnosis was made. Ninety-three studies were interpreted as showing normal kidneys. In this group there were 4 errors. In 1 a superficial (surface) cyst of the kidney was interpreted as an extrarenal mass. This could not be considered a significant error. The other 3 were serious, especially failure to demonstrate 2 carcinomas.