Improvements in HIV-RNA assays have made accurate detection of as few as 2 copies/ml possible. This study objective was the evaluation of ultrasensitive HIV-RNA quantitation (beneath current threshold: 50 copies/ml) in patients receiving different antiretroviral regimens. A cross-sectional, ultrasensitive measurement of HIV-RNA levels (detection limit: 2.5 HIV-RNA copies/ml) was performed in 154 HIV-1-infected patients receiving ARV therapy, all classed as full responders according to the 50 copies/ml cut-off. Patients were undergoing treatment with two nucleoside/nucleotide reverse transcriptase inhibitors (N/NtRTIs) plus nevirapine (NVP, n = 48), efavirenz (EFV, n = 57) or lopinavir/ritonavir (LPV/r, n = 49). Undetectable HIV-RNA (