INTRODUCTION: The measurement of flow-mediated dilatation (FMD) via ultrasound has been established as a reliable non-invasive measurement of endothelial function. However, the guidelines mention nothing regarding diurnal variation of FMD. Thus, we investigated the FMD in healthy people and diurnal variation of FMD. METHODS: Twenty-five apparently healthy persons participated in this study. All participants had no history of cardiovascular diseases, hypertension, or diabetes and used any medication. For each volunteer, the measurements were repeated in the morning and afternoon on two different days. We checked capillary blood glucose, total cholesterol, triglyceride, high-density lipoprotein (HDL), and low-density lipoprotein (LDL)-cholesterol. RESULTS: The average of FMD measurements was 8.45% +/- 2.39%. The mean values of systolic and diastolic blood pressure, heart rate, lipid profiles, and glucose levels were similar between the morning and afternoon measurements after 9-h fasting. There was no significant difference of FMD measurements between the morning and afternoon (8.32% +/- 2.27% and 8.58% +/- 2.56%, p = 0.329). Moreover, there was significant correlation between FMD in the morning and afternoon (r = 0.856, p < 0.001). CONCLUSIONS: Our study shows measurement of FMD was 8.45% in healthy Koreans. Also, there was no significant difference of FMD measurements between the morning and afternoon.