The double-blind, randomized trial will compare the effect of 24 months of supplementation with vitamin D3 and calcium to placebo on bone mineral accrual. Sixty HIV-infected children and adolescents, aged 8-16, will be randomized to receive vitamin D3 plus calcium or placebo. The primary outcome variables, BMC and BMD, will be measured before, and 1 and 2 years after randomization. Serial measurements of serum 25(OH)D, 1,25(OH)2D, PTH, markers of bone turnover will be assessed before and at defined intervals during the study.