The objective of this study was to compare epidemiological data and clinical presentation of community-acquired pneumonia (CAP) caused by Streptococcus pneumoniae, Legionella pneumophila or Chlamydia pneumoniae. From May 1994 to February 1996, 157 patients with S. pneumoniae (n = 68), L. pneumophila (n = 48) and C. pneumoniae (n = 41) pneumonia with definitive diagnosis, were prospectively studied. The following comparisons showed differences at a level of at least p < 0.05. Patients with S. pneumoniae pneumonia had more frequently underlying diseases (HIV infection and neoplasm) and those with C. pneumoniae pneumonia were older and had a higher frequency of chronic obstructive pulmonary disease (COPD), while L. pneumophila pneumonia prevailed in patients without comorbidity, but with alcohol intake. Presentation with cough and expectoration were significantly more frequent in patients with S. pneumoniae or C. pneumoniae pneumonia, while headache, diarrhoea and no response to betalactam antibiotics prevailed in L. pneumophila pneumonia. However, duration of symptoms > or = 7 d was more frequent in C. pneumoniae pneumonia. Patients with CAP caused by L. pneumophila presented hyponatraemia and an increase in CK more frequently, while AST elevation prevailed in L. pneumophila and C. pneumoniae pneumonia. In conclusion, some risk factors and clinical characteristics of patients with CAP may help to broaden empirical therapy against atypical pathogens until rapid diagnostic tests are available.