This cross-sectional study of 400 sera from a randomly selected adult population in Northern Ireland, using a microimmunofluorescence assay, demonstrated high overall seropositivity (70%) for IgG Chlamydia pneumoniae antibodies in developed populations. Seropositivity was shown to be unrelated to gender, age or smoking but there was an inverse trend between infection and educational level achieved as a measure of socio-economic status. IgG levels were also higher during the winter months suggesting seasonal variation of Chlamydia pneumoniae infection. The high prevalence of evidence of exposure to Chlamydia pneumoniae as described in this study may have implications for prevention of cardiovascular disease if further evidence conclusively determines that infection with this organism is a risk factor for cardiovascular disease.