Abstract: Objective To assess whether the T~ -786 →C mutation of endothelial nitric oxide synthase (eNOS) gene is associated with endothelial dysfunction in type 2 diabetes. Methods A total of 162 type 2 diabetic men without angiopathy were studied. PCR/alleles-specific oligonucleotide (ASO) probes were used to analyse the T~ -786 →C mutation of eNOS gene, and high resolution ultrasound was used to measure endothelium-dependent arterial dilation (EDD). Results The EDD among subjects with T/C or C/C was 3.73%±0.50%, which was significantly lower than that in subjects with T/T(4.15%±0.49%)(P<0.01). On stepwise multiple regression analysis, EDD was negatively correlated with C allele in all patients (P=0.001). EDD in smokers with T/C or C/C was significantly lower than that in smokers with T/T (P<0.05) but not in nonsmokers. The stepwise multiple regression analysis revealed that the presence of C allele was an independent determinant for reduced EDD in smokers (P<0.001), but not in nonsmokers. Conclusions The C allele of T~ -786 →C mutation of eNOS gene is a genetic risk factor for endothelial dysfunction in type 2 diabetic patients, especially in smokers.