OBJECTIVE--To determine whether Helicobacter pylori infection is associated with the development of ischaemic heart disease and whether such infection can explain the social class inequality in ischaemic heart disease. DESIGN--Cardiovascular risk factor levels, prevalence of ischaemic heart disease (Rose questionnaire angina, and/or a history of myocardial infarction), and serum antibodies to H pylori (enzyme linked immunosorbent assay) were assessed in a cross sectional population based survey. SETTING--Belfast and surrounding districts, Northern Ireland. PARTICIPANTS--1182 men and 1198 women aged 25-64 years randomly selected from the Central Services Agency's general practitioner lists. MAIN OUTCOME MEASURES--The relation of H pylori infection with cardiovascular risk factors and ischaemic heart disease. The association of social class with ischaemic heart disease. RESULTS--Systolic and diastolic blood pressure, plasma viscosity, and total cholesterol were not associated with H pylori infection. A weak negative association existed between H pylori infection and fibrinogen (mean (SE) difference in fibrinogen between infected and uninfected individuals -0.09 (0.04) g/l, P = 0.02) and between infection in women and high density lipoprotein (HDL) cholesterol (mean (SE) difference in HDL cholesterol between infected and uninfected individuals -0.06 (0.02) mmol/l, P = 0.006). A potentially important association was demonstrated between H pylori infection and ischaemic heart disease but this did not reach statistical significance (odds ratio (95% confidence interval (CI) 1.51 (0.93 to 2.45), P = 0.1). Social class was associated with ischaemic heart disease independently of cardiovascular risk factors and H pylori infection (odds ratio, manual v non-manual (95% CI) 1.82 (1.14 to 2.91), P = 0.01). CONCLUSION--H pylori may be independently associated with the development of ischaemic heart disease but if this is so the mechanism by which this effect is exerted is not through increased concentration of plasma fibrinogen. H pylori infection does not explain the social class inequality in ischaemic heart disease which exists independently of known cardiovascular risk factors.