Patients entering this study will have initiated potent antiretroviral therapy. Between 60 and 90 days [AS PER AMENDMENT 1/9/01: 60 and 104 days] after beginning their background regimen, patients are randomized to add either ABC (Arm A) or a matching placebo (Arm B) for 12 weeks. Patients completing 12 weeks of treatment continue on study for an additional 24 weeks to Week 36. Patients discontinue treatment if virologic failure occurs at any time. Patients still return to the clinic for HIV-1 RNA measurements at Weeks 12 and 36, depending on when discontinuation occurred. Patients who discontinue treatment at or after Week 12 due to virologic failure are offered open-label ABC for the remainder of the study (through Week 36). Blood samples are collected at Weeks 4, 8, 12, 20, 28, and 36. Plasma samples for population sequencing of HIV-1 PR and RT genes are collected on all patients at study entry and at the time of virologic failure. Baseline genotype (presence or absence of PR and RT resistance mutations and number of resistance mutations) is correlated to treatment outcome. Samples from the time of failure are analyzed for the accumulation of additional resistance mutations. [AS PER AMENDMENT 5/5/00: Patients and their primary care physicians will be unblinded to the patient's treatment after the study is completed at Week 36 or if virologic failure occurs at or after Week 12 [AS PER AMENDMENT 1/9/01: or if ABC hypersensitivity is suspected].]