In literature in which detection of Chlamydia pneumoniae in the artery is described, the methods used were immunocytochemistry (ICC), polymerase chain reaction (PCR), electron microscopy, and isolation. These studies demonstrated the presence of the organism in atheromatous lesions. The organism was detected frequently by ICC and PCR in atheromatous tissues (approximately 50% of subjects) but rarely in normal arteries (approximately 1% of subjects). There has been poor correlation between detection and serology. Detection studies have been used to assess the etiologic role of C. pneumoniae in atherosclerosis and to determine whether C. pneumoniae infection contributes to acute cardiovascular events. Although these studies produced suggestive evidence of an etiologic role, the use of observational studies to obtain a definitive answer is difficult. Therefore, investigators are increasingly concentrating their efforts on studies that use animal models, in vitro cultured arterial cells, and therapeutic trials in humans to determine the pathogenic role of the organism in atherosclerosis.