Chlamydia pneumoniae infection has been associated with coronary heart disease. To evaluate the mechanisms of this association, we studied whether chronic C. pneumoniae infection affects serum lipid values similarly to acute infections. Triglyceride, total and HDL cholesterol concentrations, and C. pneumoniae antibodies were measured from paired serum samples of 415 Finnish males taken 3 years apart. Chronic infection, defined as persistent IgG and IgA antibodies, was found in 20%, and the antibodies were negative (IgG < 32 and IgA < 16 in both samples) in 15% of the cases studied. The serum triglyceride and total cholesterol concentrations were higher in the subjects with a chronic C. pneumoniae infection than in the subjects with no antibodies (1.23 versus 1.03 mmol/L and 6.41 versus 6.31 mmol/L, respectively). The HDL cholesterol concentrations and the ratios of HDL cholesterol to total cholesterol were significantly decreased in the subjects with chronic infection (1.24 versus 1.36 mmol/L, P = .026; and 0.19 versus 0.22, P = .018, respectively). Chronic C. pneumoniae infection seems to be associated with a serum lipid profile considered to increase the risk of atherosclerosis. This finding supports the hypothesis that infections play a role in the pathogenesis of atherosclerosis.