Background: Bacterial pneumonia is one of the most common causes of morbidity and mortality, and accurate diagnosis and treatment of the pneumonia causative agent, especially in polybacterial cases, is difficult and much appreciated. The aim of this study was to determine the causative agents and antimicrobial susceptibility of polybacterial pneumonia in patients with lower respiratory tract infections.Methods: In this retrospective cross-sectional study, 167 cases with symptoms of lower respiratory tract infection (LRTI), admitted since March 2010 to March 2013 to Baqiyatallah Hospital, Tehran, were studied. Bronchoalveolar lavage (BAL) samples have been obtained from all these patients and have been investigated for the presence of bacterial causative agent, presence of polybacterial pattern of the infection, and the pattern of antimicrobial susceptibility testing by disc diffusion method. Also, the samples have been studied for the presence of Mycobacterium tuberculosis through culture of specific media, separately.Results: From 167 patients (62 women and 105 men), 90 cases were positive for the presence of bacterial pathogens while 77 cases were negative by culture. The incidence of bacterial pneumonia was not statistically different between men and women. Totally 117 bacterial isolates were obtained belonging to 15 different bacterial species. Mycobacterium tuberculosis (25%), Pseudomonas aeruginosa (15%) and Staphylococcus aureus (14%) were the most frequent pathogens identified. 72 percent of pneumonic cases were monobacterial infections and the others were polybacterial infections (23% two-bacterial, and 5% three-bacteria). The highest antibiotic resistance rate was seen for amoxicillin and the lowest one was seen for vancomycin.Conclusion: This study found that the prevalence of bacterial pneumonia increases with age, and also is caused by different etiologic agents. A high percentage of negative cases may be due to fastidious bacteria, viral agents, and previous antibiotic therapy. Due to high levels of resistance to antimicrobial agents, accurate diagnosis and susceptibility testing of pneumonic patients is essential.