ACTG 315 and its follow-up study, ACTG 375, were designed to examine the immunologic and virologic consequences of highly active antiretroviral therapy (HAART) in patients with moderately advanced HIV-1 disease. At the conclusion of ACTG 375, patients eligible for participation in ACTG A5136 will have received over 5 years of antiretroviral therapy. Despite long-term therapy and long-term maximal viral suppression in most patients, significant immune defects such as impaired response to antigens, including HIV, abnormally low CD4 cell counts, abnormally high immune activation, and decreased expression of CD28 persist. It is uncertain whether these defects persist as a result of irreversible damage inflicted by HIV infection or ongoing immune perturbation resulting from continuous low-level HIV replication. Cellular reservoirs of HIV that persist (despite undetectable plasma viral load) may contribute to persistent immune activation and impaired immune function. A great deal of information on the relationship between low-level viral replication and persistent immune impairment may be gained by investigating these patients before and after intensification of their antiretroviral therapy regimens.