Endothelium-derived nitric oxide formed by endothelial constitutive nitric oxide synthase (ecNOS) mediates endothelium-dependent vasodilation and antithrombotic action. We analyzed the distribution of a polymorphism of ecNOS (27-bp repeat in intron 4) in 127 ischemic stroke patients (18 with atherothrombotic, 58 with lacunar, and 51 with silent lacunar stroke) and 91 control subjects. When we assigned the four repeats as allele a, and five repeats as allele b, there was no significant difference between the genotype distribution and allele frequencies in the stroke group and in the control group (0.862 versus 0.868 for the b allele frequency). Moreover, there was also no significant difference in the genotype distribution or allele frequencies among the three stroke subgroups (b allele frequency: 0.889 for atherothrombotic stroke; 0.862 for lacunar stroke; 0.853 for silent lacunar stroke). These findings suggest that there is no overt association between this ecNOS gene polymorphism and ischemic stroke. We found no evidence that this polymorphism may be a genetic factor for the onset of cerebrovascular disease in this Japanese population.