Serum and sputum levels of ofloxacin (OFLX) were measured in 5 patients with chronic respiratory tract infections and were compared with antibacterial activity in vitro. Sputum OFLX levels higher than the MIC's against H. influenzae and K. pneumoniae were maintained during the period when daily oral administrations were continued. The MIC70 of OFLX against S. aureus was under 0.78 microgram/ml and the MIC70 of OFLX against P. aeruginosa was 1.56 micrograms/ml. The maximum OFLX levels in sputum were higher than these MIC's during the period when 200-300 mg X 2 times/day oral administrations were maintained. From these results, OFLX was considered to be effective and useful for the treatment of patients with chronic respiratory tract infections.