The objective of this study was to evaluate the plasma drug concentrations in human immunodeficiency virus (HIV)-infected pregnant women receiving highly active antiretroviral therapy (HAART) and to define the rate of occurrence of subtherapeutic concentrations for some commonly used antiretroviral drugs during pregnancy. We evaluated HIV-infected women (n = 68) in the third trimester of pregnancy in steady-state treatment with an HAART regimen administrated on a twice a day basis, which included 2 nucleoside reverse transcriptase inhibitors plus nelfinavir (NFV), lopinavir/ritonavir (LPV/r), or nevirapine (NVP). Blood samples were collected at predose (C(trough)). The following thresholds were used to define therapeutic drug concentrations-NFV: 0.8 microg/mL; LPV: 4.0 microg/mL/1.0 microg/mL (experienced/naive); and NVP: 3.1 microg/mL. At predose sampling, adequate drug concentrations were found in a higher proportion of women receiving NFV (70.8%) and LPV (75.0%) than NVP (55.6%). Median C(trough) plasma concentrations were 1.2 microg/mL for NFV, 5.5 microg/mL for LPV, and 3.1 microg/mL for NVP. Women receiving lopinavir/ritonavir had the lowest rates of detectable (>50 copies/mL) HIV RNA (15.4%) compared with rates of 22.2% and 41.7% among women receiving NVP and NFV, respectively. Genotypic resistance was detected in 50% of women with detectable HIV RNA for whom samples were available for testing. Subtherapeutic predose concentrations among HIV-infected pregnant women were more commonly found with NVP than with protease inhibitors. LPV administration was associated with the best viral load suppression.