BACKGROUND Community-acquired pneumonia (CAP) that requires hospitalization, is a common and serious disease. A permanent assessment in specialized centers, to define therapeutic guidelines according to local epidemiological factors, is mandatory. OBJECTIVE To study the etiology of CAP requiring hospital admission. METHODS Adult patients with CAP, admitted to the Clinical Hospital of the University of Chile between 1999 and 2001 were studied. A clinical, radiological and microbiological assessment was done in all patients. Blood and sputum cultures and serology for Chlamydia and Mycoplasma pneumoniae were performed. RESULTS Thirty one patients aged 62 +/- 18 years, 12 female, met inclusion criteria. An etiologic diagnosis was made in 74% of cases, (52% definitive, 22% probable). Frequency of definitive diagnosis was: 25% Mycoplasma pneumoniae, 19% Chlamydia pneumoniae and 13% Streptococcus pneumoniae. Mixed etiology was found in 16% and a final diagnosis was not obtained in 26% of the cases. No correlation was found between clinical and radiological patterns, and the etiologic agent. C reactive protein and erythrosedimentation rate were significantly higher in CAP due to Chlamydia and Streptococcus pneumoniae. CONCLUSIONS The higher frequency of Mycoplasma and Chlamydia pneumoniae, than previously reported, suggests that atypical agents should be considered in the initial antimicrobial therapy prescribed to these patients.