Objective To quantify the pulmonary penetration of piperacillin and tazobactam in critically ill patients and to investigate factors that may influence the penetration of drug into the lung. Methods The plasma and intrapulmonary pharmacokinetics (PK) of piperacillin-tazobactam in critically ill patients administered standard piperacillin-tazobactam regimens were studied. A population PK model was developed to describe plasma and intrapulmonary piperacillin and tazobactam concentrations. The probability of piperacillin exposures reaching pharmacodynamic end points and the impact of pulmonary permeability on piperacillin and tazobactam pulmonary penetration were explored. Results The median piperacillin and tazobactam pulmonary penetration ratios were 49.3 and 121.2%, respectively. Pulmonary piperacillin and tazobactam concentrations were un-predictable and negatively correlated with pulmonary permeability. Conclusions Current piperacillin-tazobactam regimens may be insufficient to treat pneumonia caused bypiperacillin-tazobactam-susceptible organisms in some critically ill patients.