A randomized double-blind, group comparative study was carried out to investigate the diagnostic efficiency and side-effect profiles, with particular reference to renal tolerance, of iopamidol and a newer non-ionic contrast medium iopromide, when used in excretory urography. The trial demonstrated that, in a population of 137 patients undergoing excretory urography and being investigated from baseline up to 2 weeks after the administration of the contrast medium, there were no significant adverse effects on the indicators currently thought to reflect renal function. The efficiency of the two contrast agents proved to be very similar and the radiographic quality of opacification was adequate in nearly every case.