Introduction. Most patients admitted to the Intensive Care Units (ICU) receive antimicrobial treatment. A proper therapeutic strategy may be useful in decreasing inappropriate empirical antibiotic treatments. When the infection is not microbiologically confirmed, the antimicrobial streamlining may be difficult. Nevertheless, there is scant information about the influence of the microbiological confirmation of the infections on empirical antimicrobial treatment duration. Method. Post-hoc analysis of prospective data (ENVIN-UCI register) and observational study of patients admitted (> 24 hours) in a medico-surgical ICU, through the three-months annual surveillance interval for a period of ten years, receiving antimicrobial treatment for treating an infection. Demographic, infection and microbiological data were collected as well as empirical antimicrobial treatment and causes of adaptation. The main goal was to establish the influence of microbiological confirmation on empirical antimicrobial treatment duration. Results. During the study period 1,526 patients were included, 1,260 infections were diagnosed and an empirical antibiotic treatment was started in 1,754 cases. Infections were microbiologically confirmed in 1,073 (62.2%) of the empirical antibiotic treatment. In 593 (55.3%) cases, the antimicrobial treatment was considered appropriate. The main cause of treatment adaptation in the microbiologically confirmed infections was streamlining (39%). The microbiological confirmation of the infection was not associated with significantly shorter empirical antibiotic treatments (6.6 ± 5.2 VS. 6.8 ± 4.5 days). Conclusion. The microbiological confirmation of infections in patients admitted to UCI was associated with a higher reduction of antimicrobial spectrum, although had no effect on the length of empirical antimicrobial therapy.