Background: Pneumonia is the third most common cause of death in the world, and mortality is highest for patients who require hospitalization. Objectives: This prospective observational study is an etiological survey of community-acquired pneumonia (CAP) over a 12-month period in the Iranian city of Mashhad. To our knowledge, this is one of the first prospective hospital-based studies to comprehensively evaluate the epidemiological, demographical, clinical, and prognostic factors of patients with CAP in Iran. Patients and Methods: We studied all adult patients (aged ≥ 16 years) with CAP admitted to Imam Reza Hospital, Mashhad, Iran, between February 2013 and January 2014. The etiological diagnosis of CAP was made through conventional culturing and staining of respiratory secretions (i.e. sputum and pleural fluid), standard BACTEC™ Plus Aerobic/F bottles for blood cultures, and the immunochromatographic assays BinaxNOW® Streptococcus pneumoniae antigen and BinaxNOW® Legionella pneumophila antigen for the detection of S. pneumoniae antigen and L. pneumophila serogroup 1 antigen, respectively. Results: Among 120 patients with CAP, the most common etiology was S. pneumoniae (24.4%), followed by Mycobacterium tuberculosis (17.5%), S. aureus (6.7%), polymicrobial agents including anaerobes (4.2%), complicated hydatid cyst (2.5%), Influenza A virus (4.2%; including 2 cases of mixed Influenza A-bacterial infection), and Klebsiella pneumoniae, Brucella melitensis, Mucor, and varicella, each in 0.8% of the patients. The diagnosis of pneumonia remained unknown in 49 (40%) patients. Conclusions: Tuberculosis was an important cause of CAP in our region. Hence, it should be considered in all patients admitted with a CAP diagnosis. Keywords: Pneumonia; Etiology; Tuberculosis; Streptococcus pneumoniae

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