This study will assess the inter-individual variability of indinavir drug levels, boosted with ritonavir, in Thai patients. Data will be collected in patients participating in the randomized study, "Monitoring Highly Active Antiretroviral Therapy (HAART) in HIV-infected patients in Thailand (PHPT-3)" (HSC 10668). The first 20 patents in PHPT-3 will have steady-state pharmacokinetic sparse sampling performed at pre-dose, and 1, 2.5, 4 and 12 hours after drug intake 1 and 2 months after initiating IDV/r 400/100 mg, twice daily. IDV/r plasma concentrations will be determined by high performance liquid chromatography. This spare PK data will be pooled with indinavir concentration data collected within an intensive pharmacokinetic studies of IDV/r [Cressey TR, et al 2005, JAC, 55, p1041-44]. Population means and variances of indinavir and ritonavir pharmacokinetic parameters were estimated using non-linear mixed effects regression models (NONMEM Version VI). The validity of the final model was evaluated using a visual predictive check (VPC) and bootstrap re-sampling techniques.