The aim of the study was to analyse the prevalence of anti-C. pneumoniae specific IgG, IgA and IgM antibodies in patients undergoing coronary artery bypass engraftment. Sera of forty-six patients aged from 40 to 74 years (mean 57.3) were examined just before the time of operation. Serum samples taken from 21 young healthy blood donors aged from 19 to 28 years (mean 22.4) and 23 sera taken from older blood donors aged 44-60 years (mean 54.7), were used as controls. Specific IgG, IgA and IgM serum antibodies were tested by the enzyme immunoassay kits (Lab-systems, Helsinki). The prevalence of anti-C. pneumoniae IgG was determined within the studied groups, to be independent of their age. On the contrary, the level and seropositivity rate of specific IgA (63.0%), as well as a co-existence of IgA and IgG (60.9%) were significantly higher in coronary patients when compared to healthy blood donors. The presence of specific IgM (positive or borderline values) were detected in 23.9% of patients, whereas they were rarer found in healthy population. In the majority of coronary patients immunological markers of C. pneumoniae infection were found.