The primary objectives of this analysis were to determine which pharmacokinetic model most accurately describes the elimination pathways for piperacillin in the presence of tazobactam through population pharmacokinetic modeling and to characterize its pharmacodynamic profile. Once the optimal pharmacokinetic model was identified, Monte Carlo simulation of 10,000 subjects with ADAPT II was performed to estimate the probability of attaining a target free-piperacillin concentration greater than the MIC for 50% of the dosing interval for 3.375 g every 6 h or every 4 h given as a 0.5-h infusion at each MIC between 0.25 and 32 g/ml.In the population pharmacokinetic analysis, measurements of bias and precision, observed-predicted plots, and r 2 values were highly acceptable for all three models and all three models were appropriate candidates for the Monte Carlo simulation evaluation. Visual comparison of the distribution of the piperacillin concentrations at the pharmacodynamic endpoint-h 3 concentrations of a 6-h dosing interval-between the simulated populations and raw data revealed that the linear model was most reflective of the raw data at the pharmacodynamic endpoint, and the linear model was therefore selected for the target attainment analysis. In the target attainment analysis, administration of 3 g of piperacillin every 6 h resulted in a robust target attainment rate that exceeded 95% for MICs of <8 mg/liter. The 4-h piperacillin administration interval had a superior pharmacodynamic profile and provided target attainment rates exceeding 95% for MICs of <16 mg/liter. This study indicates that piperacillin-tazobactam should have utility for empirical therapy of hospital-onset infections.Antimicrobial pharmacodynamics is a term used to describe the relationship between drug exposure and antimicrobial activity. In recent years there have been tremendous strides in understanding the relationship between the pharmacodynamics of beta-lactams and treatment outcomes. Beta-lactams, in contrast to aminoglycosides and fluoroquinolones, exhibit little concentration-dependent bacterial killing (7-9, 14, 24, 25, 29, 30). This phenomenon was observed in early Staphylococcus aureus time-kill curve studies, which demonstrated that the rate of bacterial killing was not improved by increasing the concentration of penicillin (19). For beta-lactams, the time during which the free (unbound) serum drug concentration exceeds the MIC of the drug for the organism (T Ͼ MIC) appears to be the best predictor of outcomes (7-9, 24, 25). The serum free-beta-lactam concentration does not have to remain above the MIC for the entire dosing interval, and the fraction of the dosing interval during which it is required to remain above the MIC for the maximal antimicrobial effect varies for the different types of beta-lactams (7-9). Although the T Ͼ MIC varies for different drug-bacteria combinations, satisfactory outcomes (near-maximal bactericidal effect) are achieved when the free (unbound)-antibiotic concentration remains above the MIC for t...