The efficacy of highly active antiretroviral therapy (HAART) is limited by the emergence of drug-resistant virus strains. Development of resistance to one or more antiretroviral drugs in a HAART regimen may result in substantial rebound viremia and require a new drug regimen. The use of virus genotyping or phenotyping to identify drugs likely to be active in an anti-retroviral experienced patient is now recommended, but these tests have limitations and their clinical utility has not yet been established. The goal of this project is to investigate the correlation between phenotype/genotype to selected antiretroviral agents and short-term change in viral load upon discontinuation of a single antiretroviral agent from a failing regimen. For the nucleoside reverse transcriptase inhibitors (NRTIs) and the protease inhibitors, resistance is poorly understood; thus, we plan to determine whether a change in viral load occurs after discontinuing these drugs. If so, we plan to correlate those changes with phenotypic and genotypic characteristics. Study participants must have a stable viral load of at least 1000 copies/ml by bDNA assay despite HAART and must be receiving one of the anti-retroviral agents designated for study:. Baseline viral load will be established by multiple bDNA sampling over a 10-day period. Next, the drug of interest will be withdrawn and the rest of the regimen maintained for either a two week (NRTIs except 3TC, protease inhibitor) or four week (3TC) discontinuation to assess for change in viral load. Finally, the withdrawn agent will be re-instituted, and serial sampling for change in viral load, genotype or phenotype will follow.