The present report constitutes an attempt to improve and modify the existing clinical evaluation method for new antimicrobial agents to treat respiratory infections. One year ago, a general guideline on the clinical evaluation of antimicrobial agents to treat respiratory infections was drafted in Japanese, leaving scope for critical discussion, and this has been translated into English, as there were no major changes. In this report, respiratory infections have been discussed under the headings "acute respiratory tract infection" and pneumonia and acute exacerbation of chronic pulmonary diseases. Standardized criteria were set for the assessment of severity of infection and effectiveness of the antimicrobial agent in question. Severity was evaluated on the basis of a combined assessment of the severity of infection and severity of the clinical condition of the patients. Clinical effectiveness of the antimicrobial agent used was evaluated on the basis of clinical outcome as well as microbiological outcome of the trial. Body temperature, local pain, cough, change in sputum quality, peripheral white blood cell count, C-reactive protein level, and chest radiograph were used as the parameters for the evaluation. To maintain the quality of specimens to be examined, Geckler's classification of specimens was used. This report was constructed based on the analysis of large amounts of material collected over the years, incorporating internal and external factors concerning the present evaluation methods. The newly suggested standardized criteria for clinical evaluation of the new antimicrobial drugs are expected to be practiced properly hereupon and subjected to further improvement if necessary.