Of 75 human immunodeficiency virus (HIV) type 1-infected patients successfully responding to 2 nucleoside reverse-transcriptase inhibitors (NRTIs) plus 1 protease inhibitor (PI), 55 started a simplified abacavir (ABC)-based triple NRTI regimen. Influences of DNA load and DNA reverse-transcriptase (RT) mutations on virological responses were assessed at month 6 after initiation of therapy. Baseline heterogeneity was observed: peripheral blood mononuclear cell (PBMC) genotyping showed 31% RT mutations with 1-5 NRTI-related mutations, 78% protease mutations had 1-5 PI-related mutations; and HIV-1-DNA levels were 1.8-3.5 log(10) copies/10(6) PBMC. Outcomes for 49 patients on a regimen of 2 NRTIs plus ABC were as follows: 22 successes, 10 blips ("blip" defined as intermittent plasma HIV-1 RNA levels between 50 and 100 copies/mL and a return to an undetectable level), and 17 failures, whereas, for patients continuing on a regimen of 2 NRTIs plus 1 PI, there were 19 successes and 1 blip. Previous treatment regimens, baseline provirus level, and PBMC genotype predicted virological outcome.