The long-term effects of childhood exercise and body mass index (BMI) on bone mass remain uncertain. We measured 1434 children, 7-15 yr of age, as part of the Australian Schools Health and Fitness Survey in 1985 and approximately 20 yr later (mean age, 31 yr). Fitness measures included a 1.6-km run and a 50-m sprint (childhood only), leg strength, standing long jump, and physical work capacity at 170 beats/min (PWC(170); childhood and adulthood). BMI was assessed at both time points. A single Sahara bone ultrasound densitometer was used to determine heel bone mass. We found, in females, there were modest but significant beneficial relationships between the childhood 1.6-km run, 50-m sprint, standing long jump, and adult bone mass. In both sexes, PWC(170) at 9 yr of age had a greater influence on adult bone mass (r(2) = 5-8%, all p < 0.05) than it did for 15 yr olds (r(2) = <1%, all p > 0.05), independent of adult performance. In the 12 yr olds, childhood PWC(170) was also associated with female adult bone mass (broadband ultrasound attenuation: r(2) = 6%, p = 0.045). In males, childhood BMI (but no performance measures) was positively associated with adult bone mass after adjustment for adult BMI. In conclusion, childhood fitness levels, particularly in females and in the early pubertal years, are predictive of adult skeletal status as measured by quantitative ultrasound, whereas BMI is predictive in males only. These results suggest that increased skeletal loading in childhood leads to an increase in peak bone mass independent of current loading.