Numerous seroepidemiological studies that suggest an association of C. pneumoniae infection and atherosclerosis have been published in last decade. The aim of this study was to assess a prevalence of C. pneumoniae antibodies in population of Zagreb area, and to investigate possible differences in prevalence of antibodies in patients with atherosclerosis and healthy controls. Forty-seven patients with coronary artery disease or myocardial infarction and 54 controls without any previous history of atherosclerosis were enrolled in the study. Sera were examined by microimmunofluorescence test. Persons with IgA antibody titers > or = 1:32, and/or IgG antibody titers > or = 1:64 were considered as seropositive. We found 75% seropositive in a total number of subjects, although number of seropositive and higher titers of antibodies were found more often in patients with atherosclerosis compared to control group: 74.5% of IgA seropositive patients versus 33.3% seropositive in control group, and 89.4% of IgG seropositive patients compared to 63% seropositive controls. Chronic (persistent) infections with C. pneumoniae were noted in 74.5% of patients and 33.3% controls.