Etravirine (TMC-125, ETV) is a second-generation non-nucleoside reverse transcriptase inhibitor (NNRTI) that demonstrates potent activity against NNRTI-resistant strains of human immunodeficiency virus type-1 (HIV-1). Thus, ETV has been used in combination with ritonavir-boosted protease inhibitor (PI) and integrase inhibitor for therapy-experienced HIV-1-infected patients. On the other hand, as ETV is a substrate and inducer of cytochrome P450 3A4 (CYP3A4), ETV may induce metabolism of PI and alter the concentrations of co-administered PIs. In order to ensure optimal drug efficacy and prevention of resistance, it is essential to monitor plasma concentrations of ETV and PIs. Here we describe the application of HPLC with UV detection for the simulataneous assay of ETV and 4 PIs, darunavir (DRV), atazanavir (ATV), ritonavir (RTV) and lopinavir (LPV). In this study, the calibration curve of each drug was linear with the average accuracy ranging from 93.6 to 110.9%. Both intra- and interday coefficients of variation for each drug were less than 11.6%. The mean recovery of all drugs ranged from 88.0 to 97.5%. The limit of quantification was 0.04, 0.04, 0.04, 0.05 and 0.07 microg/ml for ETV, DRV, ATV, RTV and LPV, respectively. These results demonstrate that our HPLC-UV method can be used for routine determination of plasma concentrations of ETV and 4 PIs in clinical settings.