A clinical study of 24 patients with respiratory tract infection due to S. aureus detected by transtracheal aspiration (TTA) was conducted, and the following results were obtained. 1) The detection frequency of S. aureus was relatively low (4.6%). 2) Pneumonia was the principal clinical manifestation. 3) Approximately one half of the patients had previously received antimicrobials. 4) Multiple organisms were frequently detected, particularly in cases of hospital-acquired infection. H. influenzae was the most frequently simultaneously detected organism. 5) On sputum examination, approximately 20 ml of purulent sputum was found in most cases, and S. aureus was detected in sputum specimens from 20 of the patients. 6) The largest number of cases was found in 1987. Onset was most common from autumn to spring. 7) Undernutrition was the most frequently associated host factor. As indicated by the above findings, the clinical expression of S. aureus infection is variable, and it is important that it be diagnosed accurately.