The use of 131I-orthoiodohippurate (OIH) scintigraphy combined with the estimated renal plasma flow (ERPF) and excretion index (EI) has been beneficial in separating impaired renal function due to graft rejection from acute tubular necrosis, ureteral obstruction, urinary extravasation and in some instances renal artery occlusion. The radionuclide data accurately identified acute and chronic rejection, confirmed by the clinical course, increase in BUN and serum creatinine and on occasion renal biopsy. Reversible and irreversible acute tubular necrosis (ATN) were clearly differentiated from acute rejection. When the ERPF and EI were plotted on a graph, multiple sequential radionuclide studies accurately predicted graft survival when chronic rejection existed. The limitation of this technique was the inability to discriminate between renal artery stenosis, ureteral obstruction and inflammatory disease. Scintigraphic studies did not distinguish between renal artery stenosis and chronic rejection. In these circumstances arteriography was the diagnostic procedure of choice. Although ureteral obstruction often can be correctly diagnosed by scintigrams, the ERPF, EI and intravenous pyelogram remained the most accurate diagnostic procedures. Recurrent glomerulonephritis, gram negative septicemia and generalized viral illness (herpes zoster or cytomegalovirus) simulated acute rejection and had to be separated by renal biopsy or the clinical course. The most valuable features of the radionuclide technique included: the noninvasive method; the simplicity; the rapidity; and the reproducibility.