This study has both cross-sectional and longitudinal components. Children with HIV infection, who are to begin taking protease inhibitors and who are already taking protease inhibitors as part of their treatment for HIV infection, will be recruited for a single cross-sectional evaluation that will include studies of lipid and glucose metabolism and body composition. In the longitudinal component, those children with HIV infection, who were studied before they began taking protease inhibitors, will be followed prospectively for 18 months to delineate further the relationships between treatment and the development of lipodystrophy and abnormalities in lipid and glucose metabolism. If the cross-sectional study does not show that the prevalence of lipodystrophy and its associated metabolic alterations increases as a function of exposure to protease inhibitors, we will terminate the prospective study. Changes in triglyceride levels, insulin levels, and regional body fat will be evaluated as primary outcome measures. As part of the evaluations performed during the cross-sectional and longitudinal studies, we will investigate the pathophysiology of dyslipidemia in HIV-infected children by assessing lipoproteins and their subclasses, apolipoproteins, and both lipoprotein lipase levels and activity.