Little is known of the effects of obesity on ertapenem drug disposition and pharmacodynamics. Thirty healthy volunteers in three body mass index (BMI) groups (10 per group), normal weight (BMI, 18.5 to 24.9 kg/m2), class I-II obesity (BMI, 30 to 39.9 kg/m2), and class III obesity (BMI, ≥40 kg/m2), were administered a 1-g dose of ertapenem. Serum concentrations were obtained over 24 h. Population pharmacokinetic data were obtained using a nonparametric adaptive grid followed by Monte Carlo simulation to determine the probability of obtaining the free drug exposure targets of the time that the free drug concentration remains above the MIC (fT>MIC) of 20% and 40% for bacteriostatic and maximal bactericidal activity, respectively. Compared to the subjects in the obese groups, area under the concentration-time curve from 0 h to infinity was significantly higher in the normal-weight subjects, whereas the total central compartment volume was higher in the class III obese subjects (P ≤ 0.05). Achieving a bacteriostatic target of fT>MIC of 20% with a 90% probability was attained at MICs of ≤0.5 μg/ml for normal-weight subjects. Class I-II and class III obese subjects were able to achieve this target only at a MIC of ≤0.25 μg/ml. For maximal bactericidal activity (fT>MIC, 40%), no group attained the target at the 90% probability level at any tested MIC. The results suggest that the standard 1-g ertapenem dose may not provide adequate drug exposure for any body mass index classification for MICs in excess of 0.25 to 0.5 μg/ml.