This is an observational cohort study of virologic and immunologic outcome after at least 48 weeks of third line antiretroviral therapy. Upto 150 children at 8 Thai sites will be enrolled. Third line antiretroviral therapy in this study is defined as an antiretroviral (ARV) regimen in a patient who has failure or intolerance to first line NNRTI-based therapy and second line PI-based therapy. Such regimens may contain new drugs or drug classes such as darunavir, tipranavir, etravirine and raltegravir The knowledge gained from this study will help the Thai government in planning its strategy to provide third line ARV therapy to children within the national program.

The primary objective of this study is to assess the virological efficacy, as measured by the proportions of children with HIV RNA below 400 and 50 copies/ml at 48 weeks after initiating third line ARV therapy.

Third line ARV therapy is defined as an ARV regimen in a patient who has failure or intolerance to first line NNRTI-based therapy and second line PI-based therapy. Such regimens may contain new drugs or drug classes such as darunavir, tipranavir, etravirine and raltegravir

Inclusion Criteria:

Children (< 18 years old) with HIV infection may enroll if one of the following criteria is met:

1. Have resistance to at least one drug in each of the 3 classes (NRTI, NNRTI and PI) and have plasma HIV RNA > 1000 copies/ml prior to switching to third line ARV therapy
2. Have intolerance to the current NRTI, NNRTI or PI treatment and need to receive darunavir, etravirine, tipranavir or raltegravir

Exclusion Criteria:

1. Have hepatic impairment with ALT ≥ 5 upper limit of normal
2. Pregnant or breastfeeding

Thai children aged < 18 years old who are on or are switching to third line antiretroviral therapy