Stratum A: 1 month to 18 months. Stratum B: greater than 18 months to 3 years. Stratum C: greater than 3 years to 8 years. Stratum D: greater than 8 years to less than 13 years. Children beginning or changing antiretroviral therapy and fulfilling the study specifications may be enrolled in the study. Children have 5 outpatient clinic visits, at entry and at 12, 24, 36, and 48 weeks, for anthropometry, body composition by bioelectrical impedance analysis, cytokine levels, viral load, CD4+ cell count, and markers of lipid and glucose metabolism.