© 2015 International Society for Hemodialysis. Vitamin D deficiency or insufficiency is highly prevalent among patients with chronic kidney disease (CKD). This study aims to determine the relationship between vitamin D and frequency of vascular access dysfunction (VAD) in hemodialysis (HD) patients. We reviewed medical records of all HD patients who had serum 25-hydroxyvitamin D (25OHD) levels at 4 outpatient dialysis facilities between January 2011 and January 2012. Patients were included if they were ≥18 years of age, had been on maintenance dialysis for ≥3 months, and had native arteriovenous fistula or synthetic polytetrafluoroethylene grafts for dialysis access. Patients with catheters were excluded. 25-Hydroxyvitamin D levels <30ng/mL were documented in 183 patients (86%). Median and interquartile range [Q1, Q3] of 25OHD level was 16 [11, 25] ng/mL. Among 213 dialysis patients, 102 had VAD. Median 25OHD level was significantly lower in patients who had VAD than in those without VAD (14.5 [10, 22] vs. 19 [12, 27.5] ng/mL; P=0.003). There was significant association between VAD and the lowest quartile relative to the highest quartile of 25OHD level. A 25OHD level <12ng/mL was associated with more than doubling of risk for VAD (OR 2.56; 95% CI [1.05-6.23], P<0.05). Of 213 patients, 140 were treated with ergocalciferol and 73 were not treated. Treatment was associated with significant reduction in VAD (OR=0.36; 95% CI [0.19-0.68], P=0.002). Vitamin D deficiency or insufficiency is an independent risk factor for VAD in HD patients; its treatment with ergocalciferol is associated with decreased VAD.