Because of previous exposure to nevirapine or other non-nucleoside reverse transcriptase inhibitors (NNRTIs), either by direct treatment or through their mothers in pregnancy, infants must often receive an alternate antiretroviral regimen that includes LPV/r. Dosing of LPV/r is currently based on a child's specific weight, and calculations of proper dosages are often too complicated to be practical in busy clinics, particularly those in limited resource settings. In order to simplify medication delivery and reduce prescribing errors, the WHO has released a dosing schedule for LPV/r based on groupings of infants and children by weight. This study will evaluate the pharmacokinetics, safety, and tolerance of LPV/r dosed according to these guidelines. The following strata were used to guide accrual: