INTRODUCTION: We compared biological outcomes in antiretroviral-naive patients with viral load (VL) > 5,000 copies/ml starting combivir-based, three-drug highly active antiretroviral therapy regimens in 2001-2002 according to the third component, namely abacavir (ABC), nelfinavir (NFV), indinavir/ritonavir (IDV/r), lopinavir/ritonavir (LPV/r), nevirapine (NVP) or efavirenz (EFV). METHODS: We evaluated virological response (HIV RNA < 500 copies/ml) and immunological response (increase of > or = 50 CD4+ T-cells/microl) separately in patients with baseline VL < 100,000 copies/ml (n = 992) and a100,000 copies/ml (n = 1,048). Hazard ratios (HR) were calculated with Cox models stratified for quintiles of propensity scores, estimated by multinomial regression from baseline characteristics. RESULTS: Median follow up was 19 months. EFV had better virological efficacy than NFV and IDV/r among patients with baseline VL < 100,000 copies/ml, with respective HRs of 0.71 and 0.72, compared with 0.81 for NVP, 0.89 for ABC and 0.99 for LPV/r. The immunological efficacy of EFV was lower than that of LPV/r (1.37) and similar to that of NFV (0.96), IDV/r (0.81), NVP (1.08) and ABC (1.04). Among patients with baseline VL > or = 100,000 copies/ml, the virological efficacy of EFV was similar to that of NVP (0.90) and LPV/r (0.97) and better than that of NFV (0.62), ABC (0.75) and IDV/r (0.78). The immunological results found in these patients were similar to those observed in patients with baseline VL < 100,000 copies/ml. CONCLUSIONS: For first-line therapy, in this observational setting, EFV, LPV/r and NVP, when added to the combivir backbone, were more likely to drive viral load < 500 copies/ml. LPV/r showed the best immunological effectiveness.