In a prospective study of a diagnositc scheme for evaluating renal masses, it was observed that a significant number of masses (29 per cent) would have been undetected without the aid of routine tomography. The renal masses detected with the aid of tomography are either small or peripheral lesions which cause no impingement upon the renal collecting system. Without specific symptoms referable to a renal mass and without hematuria, many renal masses will be overlooked on standard excretory urography. The general improvement in diagnostic quality, the ease with which this procedure may be added to excretory urography, and the minimal expense involved would be additional factors which should justify the use of routine tomography with excretory urography in any radiology department.