Rationale and Objectives. We compared the safety and efficacy of iodixanol, a nonionic, dimeric, iodinated contrast agent, with that of iohexol, a monomeric agent, in adult excretory urography. Methods. The study used a randomized, double-blind, parallel-comparison design to evaluate image quality, vital signs, laboratory values, and adverse reactions. Seventy-five patients from each of two centers were included. One third received iodixanol at 270 mg I/ml, one third received iodixanol at 320 mg I/ml, and one third received iohexol at 300 mg I/ml. The dose for every patient was 1 ml/kg of body weight. The radiographs were interpreted and evaluated for quality by the primary investigator at each center. There were no important differences among the three groups in terms of demographics. Results. Evaluation of each phase of the radiographic study showed almost uniform good-to-excellent opacification, with no significant differences among the three agents. No significant differences existed for the three groups in terms of vital signs, hematology, blood and urine chemistry, and injection discomfort. There were no deaths or serious reactions during the study, nor was there a statistically significant difference among the number of adverse events for the three agents. Conclusion. Iodixanol at both 270 and 320 mg I/ml proved to be a safe and effective contrast material for intravenous urography. In this study, its profile was essentially indistinguishable from that of iohexol at 300 mg I/ml.