A prospective study of 340 patients admitted to an acute geriatric male ward for a 12-month period was carried out to determine the aetiology and incidence of lower respiratory tract infection (LRTI) acquired either prior or after admission to the hospital. Over the year, 120 patients (35%) were found to have LRTI; a specific aetiology was found in 44 (37%) patients. The pathogens most frequently isolated were Haemophilus influenzae (11 patients), Streptococcus pneumoniae (9 patients) and influenza A (4 patients). A further 8 (7%) patients had antibody titres sufficiently raised to indicate a recent infection (influenza A6, Chlamydia psittaci 2). A marked difference in the aetiology between community- and hospital-acquired infection was demonstrated. Cross-infection was not found, although patients with influenza A infection were admitted to the ward. Mortality showed a high correlation with current and ex-smokers.