The role of Chlamydia pneumoniae in atherosclerosis is at present controversial and neglected by researchers studying the pathogenesis of the disease. Numerous seroepidemiological studies have shown an association of C. pneumoniae infection with coronary heart disease and recent studies have indicated the ubiquitous presence of C. pneumoniae in the atherosclerotic lesions. The majority of cardiologists are, as yet, more apt to consider the finding of C. pneumoniae in atherosclerotic lesions as benign particles deposited from the circulation into preformed lesions rather than as a known pathogenic Gram-negative bacterium participating in or initiating an inflammatory process. The latter possibility would result in the oxidation of lipoproteins and the induction of cytokines and proteolytic enzyme production. These phenomena are typical for atherosclerosis. Chlamydia are amenable for treatment with antibiotics, and the therapeutic importance of the verification of bacterial involvement could be significant.