Between July 1992 and June 1993, 61 patients with severe community-acquired pneumonia were admitted to our semi-intensive care unit. For all patients chest X-ray, blood gas analysis while breathing room air, Gram stain and culture of bronchoaspirate, determination of acute and convalescent anti-body titers for Legionella pneumophila, Mycoplasma pneumoniae and Chlamydia pneumoniae, blood culture when body temperature was greater than 38 degrees C, and pharyngeal swab for C. pneumoniae detection by means of an indirect immunofluorescence test were obtained. Among the patients enrolled, 15 suffered from chronic obstructive pulmonary disease, 18 had serious chronic diseases, 9 were immunodeficient and 15 had cardiovascular diseases, and only 4 had no underlying disease. Etiologic diagnosis was reached in 30 cases (49%). As expected, due to the high rate of seriously ill patients, gram-negative pathogens were identified most commonly (15%), followed by Streptococcus pneumoniae (10%) and, surprisingly, by C. pneumoniae (10%). These data, showing the possible emergence of Pseudomonas aeruginosa and C. pneumoniae, warrant further studies in order to verify whether the epidemiological pattern of severe community-acquired pneumonia is actually changing.