We will assess the relationship these variables have to TBBMC in HIV-infected children and adolescents. This will be conducted through analysis of data obtained from a sample of at least 60 HIV-infected children and adolescent being screened for the enrollment in the longitudinal clinical trial. Within the HIV positive group, linear regression will be used to determine whether HIV-related factors including antiviral medications, markers of immunodeficiency (CD4count), HIV viral load, and levels of cytokines play a role in predicting serum vitamin D levels and bone mass. Additional co-variates that will be included in the model include age, sex, race, height, weight and pubertal stage.