The aim of this paper is to review the current data concerning the associations between Chlamydia pneumoniae and heart disease. Chlamydiae have been placed in their own order, Chlamydiales, with one family, Chlamydiaceae, and a single genus Chlamydia. Molecular evaluation of rRNA sequences confirms that chlamydiae are eubacteria with very distant relationships to other eubacterial orders. The genus Chlamydia consists of four major species, Chlamydia trachomatis, C. psitacci, C. pneumoniae. and C. pecorum. These microorganisms are obligate parasites that replicate in vacuole in the cytoplasm of eukaryotic cells. Chlamydia spp. cause a variety of diseases and are mostly prevalent pathogens causing sexually transmited diseases. Since the known Chlamydia spp. are obligate intracellular parasites, pure cultures are not available for studying of many of their biochemical properties. Molecular sequence provides a comprehensive outline of the phylogenetic organization of the Chlamydiae. Chlamydiae are known to cause various heart diseases. Endo-, myo- and pericarditis were associated with chlamydial infections during the first epidemics of psittacosis due to Chlamydia psittaci. The most common Chlamydia of humans, C. pneumoniae has also been incriminated in these diseases. Recently, greatest attention has been afforded to the association of C. pneumoniae with atherosclerosis and coronary heart disease (CHD).