In this randomized, placebo-controlled study, patients are allowed to switch at entry to d4T plus 3TC or d4T plus ddI based on investigator and patient preference. Patients are stratified based on the number of nucleoside reverse transcriptase inhibitor (NRTI) treatments that are changed at entry and on screening HIV-1 RNA (obtained within 30 days of entry) as follows: switched 1 NRTI and 10,000-50,000 copies/ml vs switched 1 NRTI and greater than 50,000 copies/ml vs switched 2 NRTIs and 10,000-50,000 copies/ml vs switched 2 NRTIs and greater than 50,000 copies/ml. Patients are randomized within each of these strata to 1 of the following treatment arms: