Community acquired pneumonia (CAP) is a common and well known disease, however there is no definite agreement on a common diagnostic-therapeutic strategy. To evaluate the usefulness of microbial investigations in the clinical practice we performed a prospective study on 93 consecutive patients with a diagnosis of CAP. Group I consisted of 46 patients that underwent a diagnostic protocol including sputum, blood cultures and detection of specific antibodies against M. pneumoniae, adenovirus, respiratory syncytial virus, and L. pneumophila. Group II consisted of 47 patients, in which only sputum samples were collected and cultured. No significant differences concerning the aetiologic diagnosis, the outcome and the length of hospitalization were observed in the two groups. The aetiological diagnosis was obtained in 17 patients (18.3%). As result of information obtained from microbiol tests, antibiotic therapy was changed only in 6 patients. Among the prognostic factors only a low albumin level was correlated with the length of hospitalization (p less than 0.01). From our data, the detection of microbial aetiology should not be routinely performed in patients with CAP, but should be reserved only to the severe forms.