Increasing evidence has shown that individuals with Parkinson's disease (PD) have lower levels of 25-hydroxyvitamin D (25[OH]D) than healthy controls. Low vitamin D has been associated with endothelial dysfunction which may play a role in the pathogenesis and progression of PD. Flow-mediated dilation (FMD) is widely used as a clinical marker of overall endothelial function. We evaluated the relationship between serum 25(OH)D levels and FMD in PD. We enrolled 81 patients with early PD and 52 healthy controls, and we evaluate endothelial function based on vitamin D status and identify the association between FMD and vitamin D status in patients with early PD. The mean serum 25(OH)D levels were significantly lower in the PD patients than in the controls (21.8 ± 9.5 vs. 25.2 ± 9.3 ng/mL, p < 0.05). FMD was significantly lower in the PD patients (7.1 ± 1.8 %) than in the controls (8.1 ± 2.1 %, p < 0.05). The serum 25(OH)D was significantly associated with FMD independently of age, cardiovascular disease risk factors, body mass index, motor Unified PD Rating Scale status and homocysteine levels (adjusted R (2) = 0.331, β = 0.494, p < 0.001). These findings provide evidence of a possible association between endothelial dysfunction as assessed by FMD and low vitamin D status in patients with early PD.