A repeated measures analysis of covariance will be used to analyze the bone accrual data. Differences in bone mineral content between the treatment and placebo groups will be compared at 1 and 2 years while controlling for any possible differences between the groups at entry to the trial. The between-group main effect (vitamin D3 and calcium versus placebo) will test whether there is an overall difference between the groups, while the within-group x time interaction effect will test whether the difference between the groups is constant over the two follow-up years. The magnitude of change in the vitamin D3 and calcium group will also be compared against normative data from the BMDCS to determine how many children in each group achieve levels within normal limits (95 % CI) using the algorithm presented in preliminary studies section C.5.