From October 2003 to September 2004, we collected the specimen from 399 patients with lower respiratory tract infections in 12 institutions in Japan, and investigated the susceptibilities of isolated bacteria to various antibacterial agents and patients' characteristics. Of 474 strains that were isolated from specimen (mainly from sputum) and assumed to be bacteria causing in inflammation, 469 strains were examined. The breakdown of the isolated bacteria were: Staphylococcus aureus 76, Streptococcus pneumoniae 81, Haemophilus influenzae 84, Pseudomonas aeruginosa (non-mucoid) 56, P. aeruginosa (mucoid) 11, Klebsiella pneumoniae 36, Moraxella subgenus Branhamella catarrhalis 24, etc. Of 76 S. aureus strains, those with 2 microg/ml or less of MIC of oxacillin (methicillin-susceptible S. aureus: MSSA) and those with 4 microg/ml or more of MIC of oxacillin (methicillin-resistant S. aureus: MRSA) were both 38 strains (50.0%). Against MSSA, imipenem had the most potent antibacterial activity and inhibited the growth of all the strains at 0.063 microg/mL. Against MRSA, vancomycin showed the most potent activity and inhibited the growth of all the strains at 2 microg/mL. Arbekacin also showed the potent activity and inhibited the growth of all the strains at 4 microg/mL. Carbapenems showed the most potent activities against S. pneumoniae and inhibited the growth of all the strains at 0.125-0.5 microg/mL. Cefozopran (CZOP) also had a preferable activity (MIC90:2 microg/ mL) and inhibited the growth of all the strains at 4 microg/mL. In contrast, there were high-resistant strains (MIC: 128 microg/mL or more) for cefaclor (11.1%), erythromycin (43.2%), and clindamycin (40.7%). Against H. influenzae, levofloxacin showed the most potent activity and inhibited the growth of 83 of all the strains (98.8%) at 0.063 microg/mL. Tobramycin showed the most potent activity against P. aeruginosa (both mucoid and non-mucoid) and its MIC90 was 2 microg/mL. The activity of CZOP also was preferable and its MIC90 was 4 microg/mL for the mucoid-type and 8 microg/mL for the non-mucoid type. CZOP was the most potent activities against K. pneumoniae and inhibited the growth of all the strains at 0.125 microg/mL. Also, all the agents generally showed potent activities against M. (B.) catarrhalis and the MIC90 of them were 4 microg/mL or less. The approximately half the number (54.1%) of the patients with respiratory infection were aged 70 years or older. Bacterial pneumonia and chronic bronchitis accounted for 46.1% and 30.6% of all the respiratory infection, respectively. The bacteria frequently isolated from the patients with bacterial pneumonia were S. aureus (18.6%) and H. influenzae (18.1%). In contrast, S. aureus (16.9%) and S. pneumoniae (14.9%) were frequently isolated from the patients with chronic bronchitis. Before the drug administration, the bacteria frequently isolated from the patients were S. pneumoniae (20.6%) and H. influenzae (21.5%). The bacteria relatively frequently isolated from the patients treated with cephems or macrolides were P. aeruginosa, and S. aureus was relatively frequently isolated from the patients treated with quinolones.