Sonographic measurement of renal length is used commonly to evaluate growth of the kidneys in children. However, no previous studies have been performed to determine the degree of interobserver and intraobserver variability in such determinations. We measured the interobserver and intraobserver error in sonographic measurements of renal length obtained independently by three experienced imagers in 21 children (41 kidneys). The mean interobserver variation between any two imagers ranged from 3.87 to 5.49 mm. The mean intraobserver variation was 0.87 to 3.61 mm. The observed variability in sonographic measurement of renal length is comparable to the expected annual increase in length of the kidneys during childhood (2.2-5.7 mm per year). Therefore, caution is suggested when using sonography to evaluate renal growth in children during a year's time.