Leg-length discrepancy was determined by real-time ultrasonography (ultrasound) in 45 patients, and the measurements were compared with those of erect-posture radiography. A special device for holding and moving the ultrasound transducer was constructed, and the leg length was measured as the highest level of the femoral head in the standing position. The correlation coefficient r between ultrasound and radiography was 0.94, the mean difference was -1.9 mm, and the limits of agreement (mean +/- 2 SD) were -9.1 to 5.3 mm. The mean difference between examiners 1 and 2 was 1.7 mm, and the 95 percent confidence interval was +/- 7 mm. We conclude that leg-length discrepancy can be reliably determined by ultrasound, although the accuracy is less than that obtained by radiographic methods. Because ultrasound is not limited by radiation hazards, our technique can be used for clinical screening.