In this study we examined a possible association between a 27 base pair (bp)-repeat polymorphism in intron 4 of the ecNOS gene and myocardial infarction (MI) in a subgroup of the Turkish population. We compared MI and control groups for the frequencies of the ecNOS alleles and their genotypes. The frequency of the ecNOS 4a/a and 4a/b genotypes was found to be significantly higher in the MI group than in the control group. Interestingly, the frequency of the ecNOS 4a/b polymorphism was found to be significantly higher in the selected MI group (patients with no known secondary risk factors) than in the control and non-selected MI group. We found that the patients with MI had the frequency of the a/a genotype 4.3%, of the a/b genotype 26.6% and the b/b genotype 69.1%, The controls, however, showed only 0.6% for a/a, 18.0% for a/b and 81.4% for the b/b genotype (P<0.001; χ(2)=13.626). In this study, we show that myocardial infarction is associated with one subtype of ecNOS gene polymorphism.