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To derive and internally validate a clinical prediction rule for virologic response based on CD4 cell count increase after initiation of HAART in a resource-limited setting.A retrospective cohort study at two HIV care clinics in Gaborone, Botswana. The participants were previously treatment-naive HIV-1-infected individuals initiating HAART. The main outcome measure was a plasma HIV-1 RNA level (viral load) ≤ 400 copies/ml (i.e. undetectable) 6 months after initiating HAART.The ability of CD4 cell count increase to predict an undetectable viral load was significantly better in those with baseline CD4 cell counts ≤ 100 cells/μl [area under the ROC curve (AUC), 0.78; 95% confidence interval (CI), 0.67–0.89; versus AUC, 0.60; 95% CI, 0.48–0.71; P = 0.018]. The sensitivity, specificity, and positive and negative predictive values of a CD4 cell count increase of ≥ 50 cells/μl for an undetectable viral load in those with baseline CD4 cell counts ≤ 100 cells/μl were 93.1, 61.3, 92.5 and 63.3%, respectively. Alternatively, these values were 47.8, 87.1, 95.0 and 24.5%, respectively, if a increase in CD4 cell count of ≥ 150 cells/μl was used.CD4 cell count increase after initiating HAART has only moderate discriminative ability in identifying patients with an undetectable viral load, and the predictive ability is lower in patients with lower baseline CD4 cell counts. Although HIV treatment programs in resource-constrained settings could consider the use of CD4 cell count increases to triage viral load testing, more accurate approaches to monitoring virologic failure are urgently needed.