HIV infected subjects receiving antiretroviral treatment (lopinavir/ritonavir, stavudine, and lamivudine) for greater than 6 years through the Abbott M97-720 protocol with be evaluated for clinical lipoatrophy and mitochondrial function prior to switching nucleoside analogues from stavudine (d4T) to tenofovir and after 48 weeks or tenofovir treatment. Subjects with or without lipoatrophy are eligible for the study.