This is a single arm study to determine whether a novel dose adjustment strategy that individualizes protease inhibitor dosing to maintain drug concentrations above virologic-based threshold values (TDM) in pediatric patients with HIV is able to result in a potentially clinically useful proportion of patients who achieve a targeted inhibitory quotient (IQ). The study will consist of up to 34 children between 0 and 21 years of age (minimum weight 10 kg). Patients will have viral resistance testing performed at baseline and that information, combined with treatment history analysis and drug tolerability issues will be used to design a combination antiretroviral regimen. After the new regimen is started, pharmacokinetic monitoring will guide dose adjustments of protease inhibitors. An algorithm will be followed to make dose adjustments based upon viral phenotype and drug levels. The primary outcome measure will be the fraction of patients who attain adequate protease inhibitor levels above target values. Secondary measures will include virologic and immunologic benefits and evaluation of toxicity and tolerability.