Little is known of the inflammatory characteristics of acute infections of the respiratory tract caused by virus and unusual bacteria such as Chlamydia pneumoniae. A case is reported in whom inflammatory indices in sputum were used to investigate, for the first time, the airway inflammation during an episode of acute bronchitis caused by C pneumoniae. The patient presented with a dry cough of five days duration. C pneumoniae was identified by polymerase chain reaction (PCR) in a nasopharyngeal swab collected on day 5. Virological studies were negative. Clinical and inflammatory indices in induced sputum were measured on days 6, 8, and 11. The cough cleared spontaneously by day 11. Forced expiratory volume in one second was normal throughout. Sputum findings identified intense airway inflammation characterised by increased total cell and lymphocyte counts followed by an increase in neutrophils and a decrease in the CD4/CD8 ratio, activation of CD8 lymphocytes, and exudation as indicated by an increase in fluid phase fibrinogen. These observations suggest that sputum might be useful to monitor an inflammatory/immune response of the airway in acute infections.