The clinical assessment of a transplanted kidney is often difficult, especially in the immediate postoperative period. The biochemical parameters used to monitor renal function change slowly and can take several days to reflect the actual renal status. We have modified a technique for determining the glomerular filtration rate (GFR) from a Tc-99m DTPA renal scan and have found that it correlates with the actual GFR throughout the postoperative course. In addition, we describe a method for changing dose calibrator measurements into administered counts. This technique for determining the GFR provides a quick and accurate assessment of renal function and is useful to guide therapeutic decisions.