The problem of the hygienic state of air-conditioning installations in many countries is still treated with limited attention and, in general, this issue is disregarded both when systems are designed and when the maintenance of already operating system is performed. In order to highlight problems resulting from contamination of the installation, microbiological assessment of air conditioning systems in a hospital was carried out. The measurements particularly pointed to increased microbiological contamination of air and dust deposited in the system just downstream of the sound attenuators. This effect was observed even in the case of a relatively new installation. Thus the results confirmed that special attention should be paid to assuring the microbiological cleanliness of sound attenuators. Regular replacement of sound attenuator material should be considered due to the danger of micro-organisms deposition and growth, leading to contamination of air supplied to rooms which, in consequence, can be hazardous to peoples health.