Successful therapy following viral rebound has been problematic. Intensification of the existing regimen by adding 1 or 2 drugs generally has been avoided. However, successfully adding new drugs to an existing regimen would be advantageous since it would expose the patient to fewer antiretroviral agents in the overall treatment course. Recent evidence suggests that a significant proportion of patients who experience viral rebound while receiving a protease inhibitor (PI) actually have viral rebound with a PI-sensitive virus. Other studies have shown that treatment decisions based on resistance assays result in better virologic outcomes. This trial examines further the effect of resistance assay-directed intensification of a PI-containing antiretroviral regimen on viral load. [AS PER AMENDMENT 04/03/01: The antiretroviral regimen need not contain a protease inhibitor.]