This study will evaluate the extent of immunoreconstitution in children receiving combination antiretroviral therapy that includes a protease inhibitor and reverse transcriptase inhibitors. The children who will be evaluated and followed in this study are those who have previously been studied on other protease inhibitor-containing anti-HIV protocols within the HIV and AIDS Malignancy Branch. This study will provide a mechanism to assess the long-term immunologic changes of potent combination therapy in this unique population and to relate this to the virologic changes. A total of 50 HIV-1 infected children will be studied. The children enrolled in this protocol will either continue their current combination of protease inhibitor and reverse transcriptase therapy or, if deemed clinically appropriate, will be changed to a new, best available combination of protease inhibitor and reverse transcriptase inhibitors. Long-term immunoreconstitution, defined as the repopulation of naive CD4+T lymphocytes, will be studied by determining the presence and extent of production of new naive (thymic derived) CD4+T cells and by the ability of patients to mount new helper T cell responses after immunization with influenza and tetanus toxoid. Expansion of T cell receptor will also be explored in subsets of enrolled patients. In addition, unforeseen toxicities attributable to the use of combination antiretroviral therapy have been recognized and described in adult patients. This study will evaluate abnormalities in lipid and glucose metabolism, changes in the distribution of body fat, and surrogate markers of cardiovascular disease risk in this cohort of pediatric patients.