- Objective: to identify the infectious aetiologies of non-pneumonic lower respiratory tract infections in hospitalised elderly patients, and to characterise the patients in terms of demographic, clinical and therapeutic variables. Design: a prospective, non-interventional, purely serologically based diagnostic study. Setting: a tertiary university hospital in southern Israel. Subjects: 133 elderly patients hospitalised for non-pneumonic lower respiratory tract infections. Methods: paired sera were obtained for each of the hospitalisations and were tested using immunofluorescence or enzyme immunoassay methods to identify 13 different pathogens. Only significant changes in antibody titers or levels between the paired sera were considered diagnostic. Results: at least one infectious aetiology was identified in 77 patients (58%). At least one of seven viral aetiologies was identified in 52 patients (39%). A bacterial aetiology was identified in 27 patients (20%) including Streptococcus pneumoniae in 24 (18%). An atypical bacterium was found in 27 patients (20%) including Mycoplasma pneumoniae in 15 (11%) and Legionella spp. in nine (7%). More than one aetiology was found in 23 patients (17%). One hundred and twenty nine patients (96%) suffered from serious chronic co-morbidity. One hundred and twenty one patients received antibiotics during their hospitalisation, 106 (80%) with a beta-lactam and 42 (31%) with another antibiotic. Conclusions: non-pneumonic lower respiratory tract infection is caused in hospitalised elderly patients by a broad spectrum of aetiological agents, primarily respiratory viruses with a significant, though lesser, prevalence of classical and atypical bacteria. Despite this distribution of aetiologies, most patients are treated with beta-lactam antibiotics. The indication for antibiotic therapy in these patients and the choice of antibiotic preparation should be addressed in further studies.