A complete medical history, physical examination, and staging of HIV disease will be performed before initiation of ART. Baseline measurements prior to initiation of ART will include CBC, blood urea nitrogen, creatinine, LFTs, CD4 cell count determination and plasma HIV-1 RNA level. Weeks 12 and 24 CD4 cell count and plasma HIV-1 RNA will obtained at scheduled follow-up. Pharmacokinetic sampling will be performed at week 4 of ART in both arms and at 4 weeks after anti-TB treatment in the co-infected group. At each sampling time, 3 mL of blood will be collected into an EDTA tube. Blood samples will be collected at times 0 2, 8, 12 and 24 hours post-dose for determination of efavirenz concentrations. Efavirenz concentrations in plasma will be measured using a HPLC-MS. After sample analysis, a data set will be constructed. Anticipated covariates included in this data set to be explored will include age, sex, weight, relevant drug-metabolizing enzyme genotype, TB therapy status (on/off), and CD4 cell count. Nonlinear mixed-effects modeling (using NONMEM, version VI) will be used to estimate pharmacokinetic parameters (CL/F, V/F, AUC, Cmin, Cmax, elimination rate constant), inter-individual error, and residual error. Results of this population analysis may be used to simulate alternative efavirenz dosing strategies in this TB/HIV co-infected population, or in relevant sub-populations with variant PK/pharmacogenetics.