['To elucidate the pharmacokinetic/pharmacodynamic (PK/PD) index that predicts colistin efficacy against Acinetobacter baumannii in neutropenic murine thigh and lung infection models, and to determine the extent of the emergence of resistance in vivo to colistin.', 'PK/PD of colistin was studied in thigh and lung infection models against A. baumannii ATCC 19606 and two multidrug-resistant clinical isolates (two of the three strains were colistin heteroresistant). Dose fractionation studies were conducted over a daily dose range of 1-160 mg/kg colistin sulphate. Bacterial burden in tissues was measured at 24 h. Non-linear least squares regression analyses were employed to determine the PK/PD index (fAUC/MIC, fC(max)/MIC or fT(>MIC)) best correlating with the efficacy of colistin in each model. Real-time population analysis profiles were conducted for tissue samples to monitor the emergence of resistance.', 'The fAUC/MIC was the PK/PD index that correlated best with efficacy in both thigh (R(2) = 0.90) and lung (R(2) = 0.80) infection models. The fAUC/MIC targets required to achieve stasis and 1 log kill against the three strains were 1.89-7.41 and 6.98-13.6 in the thigh infection model, respectively, while the corresponding values were 1.57-6.52 and 8.18-42.1 in the lung infection model. Amplification of colistin-resistant subpopulations was revealed for all strains in both models after 24 h colistin treatment.'] Is fAUC/MIC the most predictive pharmacokinetic/pharmacodynamic index of colistin against Acinetobacter baumannii in murine thigh and lung infection models?