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Nelfinavir has been recently approved as a twice-daily (BID) dose regimen, but no evaluation of the influence of this regimen change on patients' protease inhibitor exposure has been published. The aim of this study was to compare trough plasma concentrations of nelfinavir obtained under the 1250-mg b.i.d regimen with the levels achieved with the original 750-mg three-times-daily (TID) regimen in 56 HIV-infected patients. Blood samples were obtained at steady state before the morning dose of nelfinavir. Plasma levels were measured by high-performance liquid chromatography. Eleven and 45 patients were following TID and BID regimens, respectively. Trough concentrations ranged from 0.14 to 11.74 mg/L and from 0.36 to 10.57 mg/L under TID and BID regimens, respectively. Large interpatient (coefficient of variation: 153%) and modest intrapatient (45%) variabilities of nelfinavir levels were observed. Twenty-one patients (38%) and six patients (11%) had levels above and below, respectively, the trough nelfinavir range (1.0–3.0 mg/L) recommended by the manufacturer. Trough levels are not affected by the dosing regimen; they mainly reflect the important interindividual variability, while remaining fairly stable over time. Many subjects had plasma levels repeatedly outside the assumed therapeutic range. Dose adjustment based on therapeutic drug monitoring may contribute to optimizing antiretroviral therapy.