The purpose of this study was to determine the effect of patient positioning on sonographic renal measurements and to test if the patient position alters the three-dimensional shape of the kidneys. The maximum longitudinal renal length and transverse renal width and depth were measured in the supine and prone position in 100 children (200 kidneys). Age ranged from 6 months to 16 years (mean age 5 years). The results were compared for statistically significant differences. The maximum measured longitudinal renal length was statistically significantly larger in the supine than in the prone position (supine position, left: 8.0 cm; right: 7.7 cm; prone position, left: 7.9 cm, right: 7.6 cm; P<0.001). There was no statistically significant change in the transverse diameters (width and depth, P>0.001) and renal volume ( P>0.001) in the supine vs. prone positions. Our results show that position-induced reshaping of the kidneys is unlikely to be responsible for the discrepancy in maximum longitudinal renal measurements comparing supine with prone positions. Position-dependent changes in the degree of filling of the renal calyces and pelvis as well as errors in caliper distance measurements for the different scan depths (supine vs. prone) are more likely to be responsible for the encountered differences. Consequently, we recommend to add prone renal length measurements in addition to the supine measurements. In follow-up examinations, renal length measurements should only be compared that have been collected in the same patient position.