Background: Individuals with non-dipper hypertension are at increased risk for cardiovascular complications and endothelial functions than are individuals with dipper hypertension. Also, eNOS gene polymorphism has been associated with cardiovascular disease. However, little is known about the relationship between the eNOS gene polymorphism and endothelial function. The aim of this study was to evaluate the possible relationships between eNOS gene polymorphism and endothelium- dependent vasodilation in patient with hypertension. Methods : One hundred patients with essential hypertension who were identified by office BP measurements underwent 24-h ambulatory BP monitoring. From among patients with demonstrating circadian BP variation, 35 non-dippers (22 men and 13 women; mean age 50.8짹14.8 years) and 65 dippers (40 men and 25 women; mean age 47.6짹14.2 years) were enrolled in this study. Endothelium-dependent response in the brachial artery, pulse wave velocity and carotid intima-media thickness were measured for each subject genotyped for the Glu298Asp polymorphism. Genotypes were determined by polymerase chain reaction restriction fragment-length polymorphism(PCR-RFLP) analysis. Results : The 24-h systolic BP, as well as nocturnal systolic and diastolic BPs were higher in non-dipper patients than in dipper patients. The other baseline clinical parameters were similar between the two groups. The flow mediated vasodilation was smaller(12.5짹4.6 vs. 9.9짹4.6%, p=0.016) and pulse wave velocity was increased(7.17짹1.13 vs. 8.05짹1.40 m/sec, p=0.04) in non-dippers than in dippers. There were no significant difference in carotid intima-media thickness and eNOS polymorphism between the two groups. Conclusion : These findings suggest that endothelium-dependent vasodilation is blunted and aortic stiffness is increased in non-dippers compared with patients who have dipper hypertension. However, the relationship of decreased endothelial function in non-dipper hypertension and polymorphism of Glu298Asp in the eNOS gene is uncertain.