BACKGROUND: A recent case-control study from Finland reported a strong relationship between high antibody titers to Chlamydia pneumoniae , strain TWAR, and both chronic heart disease and acute myocardial infarction. Other studies also suggested an association between C. pneumoniae infection and coronary atherosclerosis, based on the demonstration of increased serologic titers and the detection of bacteria within atherosclerotic tissue, but this association has not been yet regarded as an established theory. The objective of this study was to investigate the relationship between C. pneumoniae immnoglobulin G antibody titers and angiographically diagnosed coronary artery disease. METHOD: Coronary angiography was performed. Controls (n=89) were coronary angiographically normal cases and coronary artery disease (n=115) was diagnosed if coronary artery luminal diameter is obstructed more than 50% in more than one coronary artery. Micro-IF assay was used to measure C. pneumoniae TWAR antibodies. The sera were titrated in two-fold dilutions starting from 1 in 8, and a titre of 1 in 8 or more was judjed positive. RESULTS: The estimated risk of coronary artery disease, adjusted for age and gender, was greater among subjects with high (> or =1:128) antibody titers than with low (< or =1:8) antibody titers (relative risk, 7.9:95% confidence interval, 1.3-47.9). CONCLUSION: These results support an association between infection with >ENGLISH=Ptimes-i< C. pneumoniae and coronary artery disease only in high (> or =1:128) antibody titers.