OBJECTIVE To investigate whether genetic diversity of Helicobacter pyloriinfluences its association with coronary heart disease, and specifically whether the risk is confined to infection with the more virulent strains bearing the cytotoxin associated gene-A (cagA) antigen.

DESIGN AND SETTING Case–control study in hospital admitting unselected patients with myocardial infarction.

METHODS AND SUBJECTS Serological status for cagA and H pylori were determined in 342 cases of acute myocardial infarction and 214 population based control subjects free of clinical coronary heart disease.

RESULTS 38.0% of cases and 30.8% of controls were cagA seropositive (odds ratio 1.38, 95% confidence interval (CI) 0.94 to 2.01, p = 0.08). In subjects < 65 years old (153 cases, 153 controls), cagA seropositivity was associated with a 1.80-fold increase (95% CI 1.07 to 3.03, p = 0.02) in myocardial infarction risk, which increased further to 2.25-fold (95% CI 1.12 to 4.53, p = 0.01) in subjects < 55 years. There was no significant association of cagA status with classical coronary heart disease risk factors. H pylori seropositivity was present in 60.2% of cases and 53.7% of controls (odds ratio 1.12, 95% CI 0.83 to 1.51, p = 0.43). H pyloriseropositivity was not increased in young cases and did not show any interaction with age.

CONCLUSIONS The association of chronic H pylori infection with risk of myocardial infarction appears to be restricted to cagA bearing strains. The association is age dependent and stronger in younger subjects. Genetic heterogeneity of H pylori may explain some of the discordant findings with regard to the association of H pylori with coronary heart disease.