Background : The Antimicrobial Stewardship Program promotes interdisciplinary interventions and targeted recommendations for the proper utilization of antibiotics. In particular, the aim of the program is to avoid indiscriminate use of broad-spectrum antibiotics based on the documented literature on the significant impact of unsystematic usage of antibiotics on the distribution of antibiotic-resistant microorganisms. To improve the care process for pneumonia treatment using antimicrobial agents, institution-level guidelines were established and disseminated at the Seoul National University Bundang Hospital in April 2016. In this study, we evaluated changes in the physicians’antibiotic prescribing patterns both before-and after-the implementation of the guidelines. Methods : The electronic medical records of inpatients who were prescribed with one or more antibiotics in May 2014 (Group A) and May 2016 (Group B) were reviewed. Data on demographic characteristics, clinical outcomes, and antibiotic prescriptions were collected and the prescription records were compared both before- and after- the implementation of the guidelines. Results : A total of 180 patients were included in the study: 77 patients in group A and 103 patients in group B. The baseline characteristics of the patients were not significantly different between the two groups. Community-acquired pneumonia was the most common diagnosis in both the groups and the difference was not significant (68.8% vs. 67.9%; p=0.67). The type of antibiotic prescriptions used for empirical treatment was not different between the two groups. The most commonly prescribed empirical antibiotics were cephalosporins, with no significant difference (p=0.31). One of the most inappropriately used antibiotics was piperacillin/tazobactam and the rate of prescription was similar in both the groups (p=0.68). The rates of appropriate empirical selection of antibiotics remained unaltered between the two groups (67.5% vs. 71.8%; p=0.53). Conclusions : Implementation of the guidelines only exhibited no significant effect on the antibiotic prescribing patterns of physicians for the treatment of pneumonia. To improve the adequate use of empiric antibiotics, more active interventions and closer monitoring of the feedbacks should be additionally considered and evaluated in future studies.