The authors investigated the steady-state plasma pharmacokinetics of nelfinavir in HIV-1-infected children in a dosage of 30 mg/kg every 8 hours and 45 mg/kg every 12 hours in 12 HIV-1-infected children (aged 2.1 to 10.8 years) participating in an open-label study of nelfinavir in combination with stavudine and lamivudine. Median values of the primary pharmacokinetic parameters of nelfinavir 30 mg/kg every-8-hours (n = 8) and 45 mg/kg every 12 hours (n = 10) were, respectively, for the area under the plasma concentration-time curve over 24 hours, 90.5 and 71.9 h x microg/mL, for the trough concentration 1.9 and 1.0 microg/mL, and for the maximal concentration 6.3 and 5.2 microg/mL. Overall, a 7-fold interpatient variability was observed for the exposure to nelfinavir. Nelfinavir 30 mg/kg every-8-hours or 45 mg/kg every 12 hours in HIV-1-infected children generally resulted in plasma concentrations higher than those obtained in adults. However, due to a large interpatient variability in the exposure, individual dosage adjustments based on plasma concentrations may be necessary for both dosing regimens to ensure optimal treatment.