The Authors report the data of a retrospective study performed on 520 patients admitted to the Institute of Respiratory Diseases, University of Sassari, Italy, for community acquired pneumonia (CAP) from 1980 to 1995. The aim of this study was to investigate: the frequency of risk factors and their impact on severity of pneumonia; the frequency of pathogens and their correlation with the severity of the illness; antibiotic treatments. One or more risk factors were found in 86% of patients, while 14% had none. In 286 patients (55%) no etiological diagnosis was possible, while in 234 patients (45%) the pathogen was identified. Of the latter, 73% suffered from pneumonia caused by Gram-negative bacilli, 24% by Gram-positive organisms, 0.8% by Mycoplasma pneumoniae and 1.7% by respiratory viruses and endemic fungi. The mortality rate found was 2.69%. In this study, pneumonia caused by Gram-negative bacilli showed a plurilobar and often bilateral involvement, frequent resistance to the most common antibiotics, which required longer hospitalization (> 30 days). The high prevalence of pneumonia caused by Gram-negative bacilli can be explained by the presence in most of the patients, of serious and numerous risk factors.