Abstract —A polymorphism within the angiotensin-converting enzyme (ACE) gene may increase the risk of myocardial infarction in individuals previously thought to be at low cardiovascular risk. The mechanism through which it exerts this effect is unknown but may be due to increased angiotensin II–induced nitric oxide (NO) breakdown and/or reduced bradykinin-mediated NO release. We investigated whether endothelial function was different between different ACE genotypes. We performed a cross-sectional study comparing the endothelial function of the 3 genotypes ( II : n=25; ID : n=31; DD : n=12). Mean±SD ages of the subjects were 24±4 ( II ), 25±6 ( ID ), and 25±6 ( DD ) years. We assessed the impact of the genotypes on endothelial function and found that the DD genotype was associated with a significant blunting in endothelial-dependent vasodilatation (forearm blood flow data are presented as mean±SD ratio of blood flow in response to 3 incrementally increasing doses of each vasoactive agent in the test arm to blood flow in the control arm; the comparison is between DD versus ID versus II ; the P value is an expression of an overall difference by ANOVA, and the 95% CIs are of a pairwise comparison between genotypes): acetylcholine, 2.88±1.45 versus 3.81±1.93 versus 4.23±2.37 ( P =0.002; 95% CI [ II versus ID ], −0.19 to 0.91; 95% CI [ II versus DD ], 0.36 to 1.80; 95% CI [ ID versus DD ], 0.02 to 1.42). There was also a significant difference with the endothelial-independent vasodilator sodium nitroprusside, with values of 2.11±1.00 versus 2.55±1.36 versus 2.75±1.18 ( P <0.05; 95% CI [ II versus ID ], −0.15 to 0.51; 95% CI [ II versus DD ], 0.03 to 0.89; 95% CI [ ID versus DD ], −0.13 to 0.71), but not with verapamil. There was no effect of the ACE genotype on endothelial-dependent or -independent vasoconstrictors N G -monomethyl- l -arginine or norepinephrine. Investigating the effects of cigarette smoking on each genotype demonstrated that for II and DD genotypes, acetylcholine responses were further blunted if subjects smoked. These data demonstrate that the DD ACE genotype in a young population is associated with a blunting of stimulated endothelial NO and donated NO responses but not to non-NO vasodilators or vasoconstrictors.