The purpose of this study is to quantify the relative contributions of HIV and of antiretroviral treatment (ART) for HIV on rates of bone mineral density (BMD) loss among HIV-infected individuals who are antiretroviral naïve with CD4 cell counts above 500 cells/mm3. This will be achieved by comparing changes in hip and spine BMD from baseline through follow-up between the early ART and the deferred ART groups of the START study.