OBJECTIVE To determine, within a representative population group of men and women, whether alteration of the lipid profile might underlie the reported association between Chlamydia pneumoniae and ischaemic heart disease.

DESIGN AND SETTING Cross sectional survey in an area with a high incidence of ischaemic heart disease.

SUBJECTS 400 randomly selected participants in the World Health Organisation MONICA project’s third population survey in Northern Ireland.

MAIN OUTCOME MEASURES Stored sera were examined by microimmunofluorescence for IgG antibodies toC pneumoniae at a dilution of 1 in 64. Mean total and high density lipoprotein (HDL) cholesterol were compared between seropositive and seronegative individuals with adjustment for age, measures of socioeconomic status, smoking habit, alcohol consumption, body mass index, and the season during which blood had been taken.

RESULTS In seropositive men, adjusted mean serum total cholesterol and HDL cholesterol were 0.5 mmol/l (9.2%) higher and 0.11 mmol/l (9.3%) lower, respectively, than in seronegative men. Differences in women did not achieve statistical significance, but both total cholesterol and HDL cholesterol were higher (3.6% and 5.8%, respectively) in seropositive than in seronegative individuals.

CONCLUSIONS There is serological evidence thatC pneumoniaeinfection is associated with an atherogenic lipid profile in men. Altered lipid levels may underlie the association betweenC pneumoniae and ischaemic heart disease.