Plasminogen activator inhibitor-1 (PAI-1), a rapid inhibitor of tissue-type plasminogen activator, has been shown to be an independent risk factor for recurrent myocardial infarction (MI) at a young age. To investigate whether genetic variation in the PAI-1 gene is affecting plasma PAI-1 levels, a sample of 145 patients with an MI before the age of 45 years was genotyped for two polymorphisms at the PAI-1 locus, together with a sample of 95 healthy individuals of a similar age. All individuals were measured for plasma PAI-1 levels as well as for other fibrinolytic and metabolic risk indicators. A HindIII restriction fragment length polymorphism (RFLP) was used in this study in conjunction with a previously unreported eight-allele dinucleotide repeat polymorphism at the PAI-1 locus. The dinucleotide repeat polymorphism and HindIII RFLP were in strong linkage disequilibrium. There was no difference in the frequency of alleles of either polymorphism between patient and control groups. However, the smaller dinucleotide repeat alleles were significantly associated (p = 0.03) with higher plasma PAI-1 levels in the patient sample. This association was also apparent in the control sample but not at significant levels. Differences in regression coefficients for the effect of triglycerides on plasma PAI-1 levels suggest that triglyceride regulation of PAI-1 is genotype specific. Our data suggest that genetic variation at this locus contributes to between-individual differences in the level of plasma PAI-1, which is important in fibrinolysis and the pathogenesis of MI.