Vegetarians have lower blood pressure and lower cardiovascular mortality. Vegetarian diets may have lower cardiovascular risks through positive influence on endothelium-dependent relaxation and related functions. The objectives of this study were to assess the differences of vascular dilatory functions between middle-aged vegetarians and sex and age-matched omnivores before they develop any clinical manifestations of atherosclerosis. Twenty healthy vegetarians over the age of 50 and 20 healthy omnivores over the age of 50 were recruited for this study. Subjects with known risk factors for atherosclerosis such as hypertension, diabetes, obesity, hypercholesteremia, cigarette smoking, family history of vascular diseases, or taking any regular medication were excluded. Medical history, body weight, height, and duration of vegetarian diet were recorded. Baseline CBC, urinalysis and biochemical data such as fasting blood glucose, thyroid function, blood urea nitrogen, creatinine, serum electrolytes (sodium, potassium, chloride, calcium and magnesium), lipid profiles [total cholesterol, triglycerides, high density lipoprotein (HDL) cholesterol, low density lipoprotein (LDL) cholesterol] were obtained after a 14 h fast. Blood pressures and heart rate were recorded in supine position. Vascular dilatory functions, both flow-mediated (endothelium-dependent) and nitroglycerin-induced (endothelium-independent), were evaluated by using a non-invasive ultrasonographic method. The results show that there were no significant differences in the baseline characteristic between the vegetarians and the omnivores. There were also no significant differences in serum glucose, lipid profiles and thyroid function between these two groups. However, vasodilatation responses (both flow-mediated and nitroglycerin-induced) were significantly better in the vegetarian group and the degree of vasodilatation appeared to be correlated with years on vegetarian diets. Our findings suggest that vegetarian diets, by themselves, have a direct beneficial effect on vascular endothelial and smooth muscle function and may help to account for the lower incidence of atherosclerosis and cardiovascular mortality.