OBJECTIVE There are controversial results regarding the relationship of serum 25 hydroxy vitamin D [25(OH)D] and ulcerative colitis (UC) activity. To address this issue, the current study was designed to evaluate the correlation of serum 25(OH)D level and disease activity in patients with UC. METHODS All consecutively selected UC outpatients referred to a gastroenterology clinic were enrolled in 2014. UC was diagnosed based on clinical, colonoscopic and histologic findings by a gastroenterologist. The exclusion criteria consisted of the factors that might influence plasma 25(OH)D concentration including inadequate exposure or intake, reduced absorption and abnormal metabolism. Disease activity was assessed by 'Truelove and Witts' classification'. RESULTS Fifty UC patients with mean age of 36.98 (± 11.81) years were evaluated. Mean serum 25(OH)D concentrations were significantly higher in 'mild disease activity' group (27.06 ± 6.56 ng/ml) than 'moderate disease activity' group (11.14 ± 4.03 ng/ml). Lower serum 25(OH)D concentration was associated with an increased likelihood of exhibiting moderate disease activity [β = 0.37, 95% confidence interval (CI): 0.15-0.94, p value = 0.04]. CONCLUSIONS Lower serum 25(OH)D level might be associated with higher disease activity in UC patients with mild-to-moderate activity.