OBJECTIVES: To document the prevalence of Legionella sp., Mycoplasma Pneumoniae and Influenza A and B in older patients hospitalized for community-acquired pneumonia (CAP) of nursing-home acquired pneumonia (NHAP) and to determine risk factors associated with fatal outcome or prolonged hospital stay. DESIGN: Prospective clinical and serological study. PATIENTS: All patients with CAP of NHAP--confirmed by chest roentgenogram--admitted to a 320-bed acute care geriatric university hospital from May 1, 1988 to August 31, 1989, were included. Serological testing was performed upon admission and after 2 and 4 weeks. Relevant data concerning medical history, clinical examination, and laboratory data were recorded upon admission. Ninety-nine patients (age: 85 +/- 6.3 years, 36 male, 63 female) met inclusion criteria; 20 came from nursing homes, 79 from their homes in the community. MAIN RESULTS: Fourteen patients died during the month after admission. An etiological diagnosis could be established in 22 patients. No cases of Legionella pneumonia and one case of M. pneumoniae were detected. Seven patients had evidence of Influenza pneumonia. Nonsurvivors were more likely to have been admitted from a nursing home and to have a temperature less than 37.5 degrees C and elevated urea nitrogen (BUN). Cyanosis, involvement of upper lobes, elevated white blood cell counts, and higher percentage of band forms were associated statistically with longer treatment. CONCLUSIONS: This study confirms the low prevalence of Legionella sp. and M. Pneumoniae infection in CAP and NHAP in this age group. Risk factors as to outcome and length of treatment may be used as points to identify high risk patients, with special attention to patients coming from nursing homes, and patients with high BUN.