Participation in daily physical activity programs by young children is currently recommended as a means of promoting bone health. Results from studies of adults indicate that beneficial effects of either physical activity or calcium (Ca) intake may be apparent only when both these factors are present. Our results in infants indicate that physical activity combined with a low Ca diet may be detrimental in terms of bone mass accretion. The overall objective of this study is to determine whether Ca intake modifies the bone response to activity in young children 3 to 4 years of age.