The purpose of the present study was to investigate whether ultrasound-derived prediction equations for estimating total and regional skeletal muscle (SM) mass in adults are applicable for prepubertal children and adolescents. Ten Japanese prepubertal children and twenty-one adolescents volunteered for the study. Contiguous MRI images with a 1 cm slice thickness were obtained from the first cervical vertebra to the ankle joints as reference data. The SM volume was calculated from the summation of digitised cross-sectional areas. The regional SM volume was determined by anatomical landmarks visible in the scanned images. The volume units were converted into mass by an assumed SM density (1.041 g/cm3). Muscle thickness was measured by B-mode ultrasound at nine sites on different muscles (lateral forearm, anterior and posterior upper arm, abdomen, subscapular, anterior and posterior thigh, anterior and posterior lower leg). Total and regional SM mass was estimated using adult prediction equations. Mean values between measured and predicted total and regional segments of SM mass were not significantly different for adolescents, but were for prepubertal children. There was a relatively large range of the 95% limits of agreement both in prepubertal children and adolescents. These results suggest that the adult ultrasound-derived prediction equations are useful for estimating total and regional SM mass for adolescents at the group level, but the relatively high degree of variability suggested limited reliability at the individual level both in prepubertal children and adolescents.