 The objective of this study is to compare the pharmacokinetics of lopinavir tablets administered to pediatric patients as either whole or crushed tablets. The study is a randomized,open-label, crossover study of pediatric subjects already taking lopinavirritonavir tablets as part of their clinical care. THe investigators hypothesize that lopinavir exposure in pediatric patients will be lower after taking a dose of the tablet formulation, crushed and mixed with pudding or yogurt, as compared to the exposure after taking a dose with tablets swallowed whole. 