The authors report on their 9-year experience concerning the results of the determination of the effective plasma flow by means of the 131J-hippuran clearance in patients with chronic pyelonephritis. On account of the disagreements in literature it was our first task to establish the lower normal limit. Issuing form the parallel investigations with the classical PAH-clearance in 50 patients and in a control group of 71 patients with hearlhy kidneys 487 ml/min were taken into consideration as lower normal limit. The authors examine in 867 patients with chronic pyelonephritis whether it is possible to diagnose relatively early disturbances of the renal function in values of the hippuran clearance lower than 487. It was established that in 28.9% of these patients the ING was normal, in 45% the concentration power maintained and in 70% the rest nitrogen in the serum was not increased. These results allow the recommendation of a broader application of the hippuran clearance in patients with pyelonephritis, since by means of this method relatively earlier restrictions of the renal function can be established than by means of other functional tests (ING and Volhard's test).