AIM: To evaluate the association between Chlamydia pneumoniae (Cpn) infection and primary biliary cirrhosis (PBC).METHODS: Cpn IgG and IgM were determined by enzymelinked immunosorbent assay (ELISA) in 41 well-established PBC patients and two race-matched control groups (posthepatitis cirrhosis, n = 70; healthy controls, n = 57).RESULTS: The mean level and seroprevalence of Cpn IgG in PBC group and post-hepatitis cirrhosis (PHC) group were significantly higher than those in healthy controls (46.8±43.4 RU/mL, 49.5±45.2 RU/mL vs28.3±32.7 RU/mL;68.3%, 71.4%, 42.1%, respectively; P＜0.05). There was a remarkably elevated seroprevalence of Cpn IgM in patients with PBC (22.0%) compared to the PHC and healthy control (HC) groups. For the PBC patients versus the HCs, the odds ratios (ORs) of the presence of Cpn IgG and IgM were 2.7 (95% CI 0.9-6.1) and 5.1 (95% CI 1.4-18.5), respectively. Though there was no correlation in the level of Cpn IgG with total IgG in sera of patients with PBC (r = -0.857, P = 0.344＞0.05), Cpn IgM was related with the abnormally high concentrations of total IgM in PBC group.CONCLUSION: The results of this study do not support the hypothesis that infection with Chlamydia pneumoniae may be a triggering agent or even a causative agent in PBC, but suggest that Chlamydia pneumoniae infection probably contributes to the high level of IgM present in most patients with PBC.