Chlamydia pneumoniae is a common, ubiquitous respiratory tract agent causing, apart from upper respiratory tract infections, approximately 10% of all pneumonias worldwide. Antibody prevalence starts to rise early in life in the developing countries, but in industrialized countries they only begin to rise when the children start school. In early adulthood, antibody prevalence reaches approximately 50%, with men having greater prevalence than women. This prevalence rises toward old age. The pitfalls in seroepidemiologic studies associating C pneumoniae infection with various syndromes include the difficulties in testing for C pneumoniae antibodies, the necessity for careful choice of control patients, the frequently old age of the matched control patients, and the possible prevailing epidemic situation reflecting antibody saturation in control patients. However, more than 20 studies that used different serologic methods in different laboratories all over the world have established the association of C pneumoniae with atherosclerosis. Circulating immune complexes and immunoglobulin A antibodies appear to be the best markers of chronic infection in C pneumoniae seroepidemiologic studies.