In the Study of Myocardial Infarctions Leiden, we investigated the prevalence of three polymorphisms in the alpha- and beta-fibrinogen genes among 560 patients with a myocardial infarction and 646 control subjects. Secondly, we studied the relationships between these polymorphisms and fibrinogen activity and antigen levels. The TaqI, HaeIII and BclI polymorphisms in the fibrinogen gene were not associated with myocardial infarction. As we found an association of the rare B2 allele with fibrinogen levels and a similar, but weak, effect for the rare H2 allele, we conclude that a genetic propensity to high fibrinogen levels does not affect the risk of myocardial infarction. This is evidence against a causal role for fibrinogen levels in the aetiology of myocardial infarction.