Tc-99m mercaptoacetyltriglycine (MAG3) is a new Tc-99m renal agent that compares favorably to I-131 Hippuran in animal models, normal volunteers, and patients. Based on the fact that Tc-99m MAG3 has a much more rapid clearance than Tc-99m DTPA and a smaller volume of distribution, it was postulated that the image quality of Tc-99m MAG3 studies should be superior to scans obtained using Tc-99m DTPA, particularly in patients with impaired renal function. To test this hypothesis, Tc-99m DTPA and MAG3 images were obtained in three transplant patients during periods of stable but impaired renal function. In one study, the Tc-99m DTPA study was potentially misleading, whereas the Tc-99m MAG3 examination assessed the clinical situation correctly. In all three cases, the Tc-99m MAG3 images were superior.