Therapeutic efficacy and the treatment days for cure of imipenem/cilastatin sodium (IPM/CS) in treatmentof pulmonary infections were prospectively determined in comparison with those of β-lactams other thancarbapenems mainly ceftazidime (CAZ) or sulbactam/cefoperazone (SBT/CPZ). The overall response rate was 84.9% (62/73) in the IPM/CS group and 74.7% (56/75) in the β-lactam group, the difference not being significant. In the subjects having underlying respiratory diseases, the response rate was91.1% (41/45) and 73.9% (34/46) in the IPM/CS and β-lactam groups, respectively. In patients with infectionssecondary to chronic respiratory disease, the rate was 91.2% (31/34) in the former group and 66.7% (24/36) inthe latter group, respectively. The differences were significant for both stratified analyses.The treatment days for cure judged by the attending physician were 12.9±0.6 days in the IPM/CS group, and 14.5±0.7 days in the β-lactam group. The difference was not, however, significant. In patients with mild tomoderate infections, the treatment days for cure was 12.0±0.6 days (n=64) in the IPM/CS group and 14.3±0.7days (n=70) in the β-lactam group. In patients with underlying respiratory disease, the treatment days for curewere 11.8±0.7 days (n=45) and 14.7±0.9 days (n=46) in the IPM/CS and β-lactam groups, respectively. In patientswith infections secondary to chronic respiratory disease, the days were 11.1±0.7 days (n=34) and14.7±1.1 days (n=36), respectively. Thus, IPM/CS therapy significantly reduced the number of treatment daysuntil cure. There was, however, no significant difference between the two therapy groups in treatment of the patientswith severe infections, those without underlying respiratory disease, or those with pneumonia and/or lungabscess. The treatment days for cure were also assessed by the members of review committee taking into considerationof body temperature, leukocyte count, and C-reactive protein. As the result, it was 6.9±0.5 days in the IPM/CS and 10.3±0.7 days in the β-lactam groups, respectively, and the difference was significant. Time (days) untilcure was also compared between the two groups using survival time analysis, confirming a more rapid responsein the IPM/CS group. Although IPM/CS therapy was associated with a shorter response time as assessed by boththe attending physicians and the review committee, there were considerable differences between the results ofthese judgements. Thus, the duration of treatment with injectable antibiotics requires reevaluation in the future. No significant differences were observed between the groups with respect to parameters indicating side effectsand laboratory abnormalities. There were no severe symptoms or laboratory findings, and symptoms andchanges in laboratory values, if any resolved during the course of therapy or after the withdrawal of treatment. In conclusion, IPM/CS seems to be very useful as first-line therapy for respiratory tract infections and for shortening the duration of treatment.