OBJECTIVE: To validate the nosocomial infections surveillance system, establish its impact in morbi-mortality. MATERIAL AND METHODS: Surveillance of every single patient admitted during a one month period was done by one of us (DMG). Each possible case was discussed with two other hospital epidemiologists (SPLR, MSRF). This intensive surveillance was compared against the routinely surveillance performed by the nurses. We included all hospitalized patients between 11th July and 12th of August according to CDC (Atlanta, GA) nosocomial infections definitions. Patients were followed everyday and information about age, gender, underlying diagnosis, microorganisms responsible for nosocomial infections, hospital length of stay and mortality. RESULTS: During the study period 429 were admitted, 45 developed a nosocomial infection (cases) and 384 did not (controls). The incidence of nosocomial infections was 10.48 cases/100 discharges. The sensitivity and specificity of the surveillance system was 95.3 and 98.7%, respectively. Mortality in infected was 11.11% and in non infected was 2.4%. The average length of stay was 20 and 11 days for cases and non infected respectively (p < 0.01). Urinary tract infections were the most common NI (42%), secondary bacteremia (14 < or = %), pneumonia (11.11%) and deep surgical site infection (9.25%). The surgical wound infection rates were: 1.3%, 1.9% and 1.9% for clean, clean-contaminated and contaminated wounds. Patients with rapidly fatal diseases had an increased frequency of infections. The microorganisms most commonly isolated were Escherichia coli (28%), Staphylococcus aureus (11.11%), and Pseudomonas aeruginosa (8.6%). The level of antibiotic resistance was in average of 43% for those antibiotics tested. CONCLUSIONS: The sensitivity and specificity of the surveillance system was excellent. Patients with nosocomial infections had an increased length of stay and a higher mortality compared to those without NI. The validation of the surveillance system allows the production of trustable conclusions about nosocomial infections.