Objective To investigate the mechanisms of Chlamydia pneumoniae (C. pn)-induced foam cell formation, the expression of ATP binding cassette transporter AI ( ABCA1 ) and perexisome prolif-erator-activated receptor γ (PPARγ) were examined. Methods THP-1 monneytes were induced into mac-rophages after the addition of 160 nmol/L phorbol myristate acetate (PMA) for 72 h. THP-1-dorived macro-phages when co-cuhured 50 mg/L low density lipoprotein (LDL) were designated randomly in four groups: control (uninfected) group, C. pn infection group, rosiglitazone + C. pn infection group and rosiglitazone group. Lipid droplets in cytoplasm were observed by oil red O staining. The contents of intracellnlur choles-terol ester were detected by enzyme-flnoreseence. The expression of ABCA1, PPARγ, mRNA and protein were determined by RT-PCR and Western blot, respectively. Results C. pn down-regulated the expression of ABCA1, PPARγ at mRNA and protein levels in a concentration-dependent manner in THP-1-derived mac-rophages when co-incubated with LDL. Resiglitazone not only concentration-dependently alleviated the down-regulation of ABCA1 expression by C. pn infection (P<0.05), but also markedly suppressed the accumula- tion of lipid droplets and cholesteryl ester by C. pn at higher concentrations ( 10 and 20 μaol/L). Condu-sion C. pn induces foam cell formation by down-regulating the expression of ABCA1 via PPART pathway, which may provide a new evidence for the development and progression of atherosclerosis initiated by C. pn infection.