The intra- and interobserver variability of the measurement of renal length in an unselected series of children attending for renal tract ultrasound were estimated using a standard technique and test-retest protocol. The standard errors of measurement were calculated and the limits of agreement derived. The results indicate that for normal kidneys there is a 95% probability that a second measurement by one observer will be within 4.6 mm of his first reading. Similarly, a single measurement will be within 7.8 mm of a reading by a second examiner. For abnormal kidneys in this series the intraobserver variability was similar to normal kidneys but interobserver variability was greater. Observer error in the measurement of renal length is equivalent to between 2 and 3 years normal growth for children older than 1 year.