Seroepidemiologic studies have provided information on the association of Chlamydia pneumoniae with the classical risk factors of coronary heart disease (CHD). C. pneumoniae infections are more common in smokers than in nonsmokers, suggesting that smoking predisposes to the development of chronic infection. Infections may also affect lipid metabolism. In persons with acute pneumonia caused by C. pneumoniae, high-density lipoprotein (HDL) values are lower and triglyceride values higher than seen in pneumonia caused by viruses and other bacteria. Furthermore, chronic C. pneumoniae infection is associated with elevated triglyceride and lowered HDL levels in healthy Finnish men. Recent studies also suggest that chronic C. pneumoniae infection considerably enhances the effect of the metabolic syndrome on the CHD risk. Thus, known CHD risk factors may be partly explained by their association with chronic C. pneumoniae infection. Further studies are needed to elucidate the pathogenetic mechanisms underlying these associations.