Objective: to analyze the efficiency of prescribed antibiotic therapy in patients with viral and bacterial pneumonia of variable severity in the epidemic and postpandemic periods (October 2009 to February 2011). Subjects and methods. Case histories of 85 patients (mean age, 35.92±13.3 years) were retrospectively analyzed; out of them 31 patients were followed up and treated in intensive care units (ICU). The patients were grouped according to PaO2/FiO2: 1) PaO2/FiO2 >300 (n=54); 2) PaO2/FiO2 <300 (n=26); 3) PaO2/FiO2 < 200 (n=5). Results. Regression analysis proved the importance of antimicrobial therapy used in admitted patients having risk factors and PaO2/FiO2 > 300. Bacteriological follow-up revealed that all the patients who had PaO2/FiO2 <200 and died in ICU had been nosocomially reinfected with polyresistant microorganisms and targeted antimicrobial therapy could not be performed. Conclusion. Early antibiotic therapy in patients with PaO2/FiO2 >300 and risk factors improves the prognosis of the disease. All the patients who have PaO2/FiO2 >300 and are ineffectively treated for antibacterial pneumonia should undergo followup analysis of bronchoalveolar lavage to be switched to targeted antibiotic therapy. The patients who have PaO2/FiO2< 200 are at the highest risk for nosocomial reinfection and pose difficulties in choosing adequate antibiotic therapy. There is a significant relationship between the detection of polyresistant microorganisms and ICU mortality rates. © 2015 AVES Ibrahim Kara. All rights reserved.