Background: Chlamydia pneumoniae has been linked with atherosclerosis. While several studies have shown that C pneumoniae contributes to the acceleration of atherosclerotic lesions, studies on the initiation of atherosclerosis by C pneumoniae are sparse. The present study investigated whether C pneumoniae infection could initiate the atherosclerotic lesion development in hyperlipidemic rats. Methods: Twenty, 11-week-old OLETF rats and twenty LETO rats that were maintained on a high-fat diet were either mock-inoculated or inoculated 3 times at 2-week intervals with C pneumoniae. Atherosclerotic lesion types and mean lesion area were analysed in the aortic arch at 40 weeks. Results: Immunocytochemical stain demonstrated C pneumoniae and macrophages in atherosclerotic lesions. While no atherosclerotic lesion developement was observed in mock-infected LETO rats, early to advanced atherosclerotic lesions were found in the other groups. C pneumoniae-infected OLETF rats showed more diffuse thickening of vessel wall, advanced atherosclerotic lesions and greater mean lesion area than the other groups (91972.7 關m2 in C pneumoniae-infected OLETF rats vs 49080.5 關m2 in mock-infected OLETF rats vs 32864.7 關m2 in C pneumoniae-infected LETO rats vs 0 關m2 in mock-infected LETO rats, P < 0.0001). Conclusion: C pneumoniae infection leads to the development of atherosclerosis in type 2 diabetic and even in non-diabetic rats in concert with hyperlipidemia. These results suggest that C pneumoniae could initiate the development of atherosclerosis.