The surveillance system for Hospital Acquired Infections (HAI) implemented in the S. Eugenio hospital of Rome allows to monitor the distribution of the micro-organisms by service and their resistance to antibiotics. It is based upon the data collected by the Central Analysis Laboratory of the hospital. The data of four high-risk departments (Surgical service, Intensive Care Unit, Haematology, Burn Unit) are reported. In the period October 1992-September 1993, 3909 samples have been analyzed; 1603 (43.1%) were found positive to the microbiologic analysis. The results of the antibiotic resistance concerning four micro-organisms, agents of HAI are reported and discussed. Surveillance systems are necessary to limit the frequency of HAI.