Sputum isolates of MRSA have been on the increase, recently. Preventive measures against MRSA nosocomial infections have become important in Japanese hospitals. Clinical study was performed on 29 patients from whom MRSA was isolated more than 10(7) cfu/ml using the quantitative sputum culture method. All had a history of admission, therefore nosocomial infections caused by MRSA could very often occur. MRSA was determined as a causative organism in 3 on the basis of symptoms, laboratory data, chest X-rays, and effect of antimicrobial agents. These three patients improved by a single or combined administration of minocycline, arbekacin and/or fosfomycin. In 15 patients, MRSA was frequently isolated, but was thought to be colonized. In 3 patients, MRSA was not isolated without administration of antimicrobial agents thereafter. It was supposed that most of MRSA isolates from sputum were not the causative organism of the respiratory tract infection.