Two related studies were conducted to assess the associations between markers of skeletal modeling and remodeling in healthy children. Members of monozygotic twin pairs, aged 6-14, enrolled in a clinical trial of calcium supplementation, were studied at the end of the period of supplementation and for 3 years thereafter. Supplemented children had significantly higher rates of gain in bone mineral density (BMD) (+3% on average) during the period of supplementation accompanied by significantly lower concentrations of serum osteocalcin (OC, -15%). During postsupplement follow-up, both differences in BMD and OC disappeared. Black females, age matched to the baseline ages of the white children, had significantly lower serum concentrations of both OC and tartrate-resistant acid phosphatase (TRAP) at all ages and higher BMDs. When stratified on serum TRAP concentrations, regardless of race, children with lower concentrations had significantly higher BMDs, and no racial differences were apparent. In regression models accounting for 70-80% of the variability in BMD in children, body size and TRAP, but not race, remained significantly associated with BMD. The skeletal advantages seen with calcium supplementation and black race appear to be associated with reduced rates of skeletal turnover. Given that markers of turnover during growth reflect both skeletal modeling and remodeling, and there is no apparent advantage to reduced skeletal modeling, it seems probable that reduced remodeling is the factor that accounts for the increases in bone mass.