Our hypotheses are that (1) the increase in bone mass resulting from a physical activity program will be more pronounced in children randomized to receive a Ca supplement compared to the increase in children randomized to receive a placebo; and (2) 12 months after cessation of the activity program, bone mass will remain higher in children randomized to gross motor activity compared to children randomized to fine motor activity, and the beneficial effect of Ca supplementation will persist only among children randomized to gross motor activity. We will test these hypotheses in a randomized 2 x 2 factorial trial in 3- to 4-year-old children. We will randomize children into either a gross motor or fine motor activity program that will be conducted in childcare centers 5 days a week for 1 year. We will further randomize each child into either a Ca supplement (1 g/d) or placebo group.