Type 2 diabetes mellitus (T2DM) and hypertension (HT) commonly coexist. While endothelial nitric oxide synthase (eNOS) haplotypes have been associated with HT, it is unknown whether eNOS genotypes/haplotypes are associated with altered susceptibility to HT in patients with T2DM. We studied the distribution of three eNOS genetic polymorphisms: a single nucleotide polymorphism in the promoter region (T(-786)C), in exon 7 (Glu298Asp), and a variable number of tandem repeats in intron 4(b/a). Genotypes were determined for 102 healthy controls, 119 patients with HT, 66 patients with T2DM, and 113 patients with T2DM+HT. In addition, we also compared the distribution of eNOS haplotypes in the four groups of subjects. No differences were found in genotype and allele distribution among the four groups. Conversely, the haplotypes "C Glu b" and "C Asp b" were, respectively, more and less common in healthy controls than in HT or in T2DM+HT groups (24% versus 6% and 5%, respectively; both P<0.00625; and 8% versus 18% and 18%, respectively; both P<0.00625). Moreover, DM patients presented an overall distribution of eNOS haplotypes that was not different from healthy controls (P>0.05). Additionally, the haplotypes "C Glu b" and "C Asp b" were, respectively, more and less common in T2DM group than in T2DM+HT group (19% versus 5%; and 7% versus 18%, respectively; both P<0.00625). Our findings suggest a protective effect for eNOS haplotype "C Glu b" against the development of hypertension, and that haplotype "C Asp b" increases the susceptibility to hypertension in patients without or with T2DM.