This study was performed to investigate the intrapulmonary penetration of lascufloxacin in humans. Thirty healthy adult male Japanese subjects, allocated into five groups, received lascufloxacin in a single oral dose of 75 mg. Bronchoalveolar lavage and blood sampling were performed simultaneously in each subject at 1, 2, 4, 6, or 24 h after administration, and lascufloxacin concentrations in plasma, epithelial lining fluid, and alveolar macrophages were determined. Lascufloxacin was rapidly distributed to the epithelial lining fluid with a time to maximum drug concentration () of 1 h, which was identical to that in plasma. The maximum concentration of drug () values in plasma, epithelial lining fluid, and alveolar macrophages were 0.576, 12.3, and 21.8 μg/ml, respectively. The corresponding area under the concentration-time curve from 0 to 24 h (AUC) values were 7.67, 123, and 325 μg · h/ml. The mean drug concentrations in the epithelial lining fluid and alveolar macrophages were much higher than those in plasma at all time points examined, and the average site-to-free plasma concentration ratios fell within the ranges of 57.5 to 86.4 and 71.0 to 217, respectively. Drug levels in epithelial lining fluid and alveolar macrophages exceeded the MIC values for common respiratory pathogens. (This study was registered at JAPIC under registration number JapicCTI-142547.).