Efficacy exposure-response relationships of the CCR5 antagonist maraviroc were evaluated across two phase III clinical trials. This post-hoc analysis used 48-week efficacy data from 841 treatment-experienced patients infected with CCR5-tropic human immunodeficiency virus type 1 (HIV-1), identified by the enhanced sensitivity Trofile assay. Probability of treatment success (viral RNA <50 copies/ml) was modeled using generalized additive logistic regression, testing exposure, clinical, and virologic variables. Prognostic factors for treatment success (in decreasing order of Akaike information criterion (AIC) change) were: maraviroc treatment, high-weighted overall susceptibility to background treatment, absence of an undetectable maraviroc concentration, high baseline CD4 count (BCD4), low viral load (VL), race (other than black), absence of non-R5 baseline tropism (BTRP), and absence of fosamprenavir (FPV). No concentration-response relationship was found with treatment (maraviroc vs. placebo) and presence/absence of undetectable maraviroc concentration (adherence marker) in the model. The maraviroc doses studied (300 or 150 mg with potent CYP3A4 inhibitors once (q.d.)/twice daily (b.i.d.)) deliver concentrations near the top of the concentration-response curve.CPT: Pharmacometrics & Systems Pharmacology (2013) 2, e64; doi:10.1038/psp.2013.42; published online 14 August 2013.