OBJECTIVE: Although combinations of different antiretroviral drugs are increasingly used by pregnant HIV-1-infected women, few human data are available to evaluate in utero protease inhibitors (PI) exposure. The aim of this study was to assess the extent of transplacental passage of PI at delivery. METHODS: Pregnant women treated with antiretroviral drugs including PI and/or nevirapine were eligible for the study. Placental transfer was determined by comparison of drug concentrations in blood samples simultaneously collected from a peripheral maternal vein and the umbilical cord at delivery. Drug levels were determined by high-performance liquid chromatography. RESULTS: Thirteen maternal-cord blood sample pairs were evaluable for transplacental passage determination (nine nelfinavir, two ritonavir, one saquinavir, one lopinavir, two nevirapine). Median cord and maternal drug concentrations, respectively, were nelfinavir < 250 and 1110 ng/ml; ritonavir < 250 and 1113 ng/ml; saquinavir < 100 and 350 ng/ml; lopinavir < 250 and 3105 ng/ml and nevirapine 2072 and 2546 ng/ml. The cord-to-maternal blood ratio was extremely low for all PI. CONCLUSION: PI do not cross the placenta to an appreciable extent and consequently cannot be expected to exert a direct antiviral activity in utero during the whole dosing interval. Limited transfer may result from their high degree of plasma protein binding and their backwards transport through P-glycoprotein, largely expressed in the placenta. In contrast, nevirapine readily crosses the placental barrier. Such considerations may support treatment decisions in pregnant women.