The purpose of this study was to determine whether growth-related changes in bone properties can be detected in prepubertal boys using quantitative ultrasound (QUS) and to determine whether resistance training stimulates bone changes. Two groups, each of thirty 9-10 year-old boys, participated in regular physical education classes or in resistance training. Tibial speed of sound (SOS) (SoundScan 2000, Myriad) was assessed at the beginning of the school year and after 8 months. At baseline, there were no differences between groups in tibial SOS, anthropometric measures or pubertal development. At the end of the year, the tibial SOS increased (p<0.001) in both groups to a similar extent. In addition, there were no differences in the increases in height between the two groups. This indicates that resistance training during the physical education program did not induce changes in bone beyond what would be expected by the mere effect of growing. We conclude that changes in tibial SOS, as obtained with QUS, can be detected in groups of prepubertal boys over a period of 8 months.