Chlamydia pneumoniae has been recognized as a cause of respiratory tract infection in humans, and its prevalence has been shown to vary among different age groups and populations. The prevalence of Chlamydia pneumoniae antibody was determined by serological investigation in 343 healthy children and in 77 children consecutively hospitalised for pneumonia in southwestern Greece. Seventy-eight (22.7%) healthy children had IgG Chlamydia pneumoniae titers > or =1/8. The prevalence of Chlamydia pneumoniae antibody in the age groups 6 months-5 years, 6-9 years and 10-15 years was 7.9%, 11.4% and 36%, respectively. One child hospitalised for pneumonia had serological results consistent with acute Chlamydia pneumoniae infection. The results of the present study suggest a low prevalence of Chlamydia pneumoniae antibody among preschoolers in Greece, followed by a steep rise in children 10-15 years of age. Chlamydia pneumoniae is not a common etiologic agent of childhood pneumonia requiring hospitalisation.