BACKGROUND: Osteopontin (OPN) is potent mineralization and inducible vascular calcification inhibitor. Studies on patients with elevated OPN levels are associated with both the extent of cardiovascular disease and restenosis. Therefore, it is interesting to investigate a potential association between serum OPN levels and endothelial function among patients with kidney transplantation (KT). METHODS: Fasting blood samples were obtained from 84 patients with KT. The endothelial function and vascular reactivity index (VRI) were measured using the digital thermal monitoring test (VENDYS). Serum OPN levels were measured using a commercially available enzyme-linked immunosorbent assay. In this study, VRI of ＜ 1.0 was used as the poor vascular reactivity, VRI of 1.0-1.9 was used as the intermediate vascular reactivity, and VRI of ≥ 2.0 was used as the good vascular reactivity. RESULTS: A total of 12 patients with KT (14.3%) were categorized as having poor vascular reactivity, 29 (34.5%) as intermediate vascular reactivity, and 43 as good vascular reactivity. Elevated serum alkaline phosphatase (ALP) (ALP [P = 0.005] and serum OPN [P = 0.002]) levels were associated with poor vascular reactivity. Advanced age (r = -0.223, P = 0.041), serum ALP level (r = -0.344, P = 0.017), and serum OPN level (r = -0.427, P ＜ 0.001) were negatively correlated with VRI values in patients with KT. Multivariable forward stepwise linear regression analysis noted that serum OPN level (β = -0.427, adjusted R^2 change = 0.173; P ＜ 0.001) was significantly negatively associated with VRI values in patients with KT. CONCLUSIONS: Serum fasting OPN level was negatively associated with VRI values and increased serum OPN level was associated with endothelial dysfunction among patients with KT.