Differences in kidney depth, particularly between the two kidneys of an individual patient, may lead to distortions of the renogram owing to attenuation and geometrical factors. A method of assessing the magnitude of these variations was developed. A double isotope technique was used, employing 125-I and 131-I Hippuran, the kidney depth being a function of the ratio of the count-rates from the two isotopes. It was necessary to apply a correction for extrarenal activity, based on a third detector and doubly labelled human serum albumin. The average kidney depth observed was 7.3 cm; in only two cases out of 13 did the difference in depth of the two kidneys exceed 1 cm. It is concluded that when 131-I Hippuran is used the consequent distortion of the renogram is acceptable. With 125-I the distortion may be excessive, even though the contribution of extrarenal activity is then reduced. There was frequently an apparent increase in the depth of activity during the renogram attributable to intrarenal translocation of the tracer.