To increase the accuracy in the routine measurement of the side distribution of kidney function from 131I-Hippuran renograms, a simple method of background subtraction has been developed. Principally a background curve was adapted to the renogram on each side, assuming an initial amplitude corresponding to the amplitude of phase I of the renogram. The amplitudes of the renogram over background after 2 min is recognized as a measured of kidney function and a background quotient was calculated by dividing the background curve amplitude at 2 min with its initial amplitude. Mean values of the background quotient recorder over thorax was 0.62 and over an empty kidney region 0.73. It was found that a quotient of 0.70 could be used generally, thus omitting the need for a background curve registration at each renographic examination.