Background Aerobic exercise enhances endothelium-dependent vasodilation in healthy individuals. It is thought that exercise increases nitric oxide (NO) production and decreases NO inactivation, leading to an increase in NO bioavailability. Angiotensin II and NO have important roles in maintaining vascular tone. There are polymorphisms of the angiotensin converting enzyme (ACE) gene and the presence of the deletion (D) allele has been associated with higher concentrations of circulating and tissue ACE. In this study, the relationship between endothelial function and ACE gene polymorphisms was investigated in athletes and sedentary subjects. Methods and Results The study group comprised 56 endurance athletes and 46 sedentary subjects who underwent brachial artery ultrasonographic examination. ACE insertion (I) and D allele frequencies were analyzed in all patients. Baseline brachial artery diameter and resting blood flow were similar in athletes and controls (p>0.05). The flow-mediated dilation (FMD) was 8.48±3.65% in athletes and 5.16±2.5% in controls (p=0.0001). FMD was significantly different between ACE genotypes in the athletes (p<0.0001): it was higher in ACE II (10.5±1.6%) subjects than in the DI (8.4±2.3%) or DD (7±1.2%) subgroups. Conclusion Regular isotonic exercise can improve endothelium-dependent vasodilation especially in those with the ACE II genotype. (Circ J 2005; 69: 1105 - 1110)