Lopinavir/Ritonavir (Kaletra) PK in Children

Inclusion Criteria: * Documented HIV infection * Taking lopinavir/ritonavir (Kaletra) tablets at standard pediatric doses for greater than two weeks * Concomitant medications and/or natural products, including potentially interacting products, have been stable for greater than two weeks and are not expected to change over the course of the study * Ability to understand study procedures and assent to participate * Parental or guardian consent * Aged 6 - 17 years Exclusion Criteria: * Acute serious medical illness or infection (in the judgment of the investigator)requiring treatment and/or hospitalization within 14 days prior to study entry * Pregnancy * Concomitant medications/natural products that have been started within past two weeks and/or that will be changed over the course of the study.