Objective : The purpose of this study was to investigate the relationship between bone mass and maturation, physical activity, milk and dairy products consumption in adolescence. Subjects and methods : Healthy 668 Japanese adolescents (334 boys and 334girls) aged 9-14y were participated in this study. We measured speed of sound (SOS) and transmission index (TI) at the calcaneus by using AOS-100 (ALOKA Co., Ltd., Tokyo). Osteo sono- assessment index (OSI) was calculated with SOS and TI, percent of body fat assessed by bioelectric impedance method with TBF-300 (TANITA Co., Ltd., Tokyo). We administered a questionnaire, to survey the maturation (the growth of height for boys, the age at menarche for girls), physical activity (walking, participating in sports club and playing), milk and dairy products consumption. Results : Multiple regression analyses indicated that lean body mass, walking and participating in sports club explained about 15% of the OSI. There were no significant differences in SOS and OSI in milk and dairy products consumptions. In the stepwise multiple regression analyses, maturation and lean body mass were the most important and consistent factor for predicting SOS and OSI (attributable r2=2.2%−17.5%).