 This is a multicenter, observational cohort study. HIV-infected children from 5 institutions are followed to assess HIV disease progression: 1. Bamrasnaradura infectious disease institute,2. Faculty of Medicine Siriraj Hospital, Mahidol University, 3. Chiangrai Prachanukroh Hospital, 4. Sanpatong Hospital and 5. HIV-NAT, Thai Red Cross AIDS Research Center. 