We hypothesize that vitamin D stores, as assessed by serum 25-hydroxyvitamin D levels, are an important determinant of bone mass in HIV-infected children and adolescents receiving HAART. We will test this hypothesis by evaluating whether measurements of bone mass are associated with vitamin D stores, as measured by serum 25-hydroxyvitamin D levels and other indices of mineral metabolism, in treated HAART-treated HIV-infected children and adolescents.