Background Failure rates of second-line boosted protease inhibitor antiretroviral therapy regimens in children rise over time. Therapeutic drug monitoring (TDM) can contribute to assessments of adherence. The authors assessed the performance characteristics of the US DHHS-recommended lopinavir (LPV) concentration of 1.0 mg/L for predicting virologic failure (VF) and intermediate-to-high level LPV resistance in Asian children. Materials and Methods LPV concentration, HIV RNA level, and adherence data from study participants in Indonesia, Thailand, and Vietnam receiving second-line LPV-based ART and followed for ≥24 weeks were analyzed. Results A total of 223 children at a median age of 10.4 (interquartile range, IQR 7.9–13.4) years were enrolled, 61% were male. Their mean CD4 was 842±438 cells/mm3, and the median LPV duration was 2.5 (IQR 1.3–4.2) years. Five out of 84 (6%) and 18 out of 139 (13%) children had LPV trough and random concentrations <1.0 mg/L at study week 24. Using either of these trough or random LPV concentrations, a cutoff at 1.0 mg/L gave an area under the receiver operating characteristics (AROC) curve of 0.69 in predicting VF with sensitivity of 44% (95% CI 23–66) and specificity of 94% (95% CI 89–97). Seven of 21 with VF and resistance results available had ≥1 major protease inhibitor mutation. Multivariate logistic regression found LPV concentrations <1.0 mg/L (OR 6.47; 95% CI 2.15–19.50, P = 0.001) and CD4 ≤20% (OR 2.83; 95% CI 1.01–7.89, P = 0.05) were independently associated with HIV RNA >1000 copies/mL. No factors predicted major LPV resistance mutations. Conclusions The authors support that the DHHS target LPV concentration of <1.0 mg/L is predictive of virologic failure (VF), but not of the presence of major LPV mutations.