Objectives We explored the relationship between bone mineral density (BMD) and lifestyle in juveniles to identify factors leading to higher peak bone mass and prevention of osteoporosis in later life. Methods Juveniles (1,364 students: 770 boys and 594 girls, aged 6-18 years) attending school in Hokkaido prefecture, Japan, were asked to complete a brief selfadministered diet history questionnaire for 10-year-olds (BDHQ10y) providing information about personal history, lifestyle, and intake of nutritional elements. In addition, BMD and grip strength were measured. We analyzed the relationship between BMD and lifestyle factors. Results The difference in BMD for boys was larger among the junior and senior high school groups. The difference in BMD for girls was larger among older elementary and later school children. Anthropometric variables and grip strength were strongly correlated with BMD. Having a nap-time routine was significantly correlated with BMD, but sleep time and sports club activities were not. BMD among juveniles who attained secondary sexual characteristics was significantly higher than that of juveniles of the same age who had not attained these characteristics. Calcium intake was significantly lower in senior high school students compared with other grades. Consumption of milk by senior high school boys and junior high school girls was weakly correlated with BMD. Conclusions Our findings encourage educational interventions to counsel students to avoid weight loss and calcium deficiency. This effective intervention should begin before the higher elementary school, when juveniles have the greatest likelihood for preventing lower peak bone mass and osteoporosis.