Objective:To analyze the influence of vitamin D 3 supplementation on the clinical efficacy of mesalazine in patients with ulcerative colitis (UC). Methods:From January 2015 to December 2020, patients with mild-to-moderate active UC were retrospectively and continuously enrolled, who accepted mesalazine treatment for at least 12 months at the Second Affiliated Hospital of Wenzhou Medical University. According to simultaneous supplement of vitamin D 3 (125 IU/d), the patients were divided into study group and control group. Demographic and disease characteristics, serum 25-hydroxyvitamin D[25(OH)D] levels and other information were collected through retrieving hospital database. Student′s t-test, Mann-Whitney U test and Chi-square test were applied for comparison of disease characteristics. The changes of modified Mayo scores[ΔMayo] and 25(OH)D[Δ25(OH)D] were compared before and after treatment by paired t-test, Wilcoxon signed rank test and Chi-square test. Multiple linear regression model was used to analyze the independent factors affecting ΔMayo and Δ25(OH)D, and variables with P-values less than 0.20 in the univariate analysis were allowed for further multivariate analysis. Results:A total of 74 UC patients (44 males, 30 females), with median age (range) 39.5 (20-76) years old, were analyzed and respectively assigned into study group ( n=36) and control group ( n=38). In study group, the average level of serum 25(OH)D was significantly increased at month 12 compared with that at baseline [(22.87±7.30) μg/L vs. (18.15±7.48) μg/L, P<0.001]. However, no significant elevation of serum 25(OH)D was found in control group [(19.17±8.49) μg/L vs. (19.82±9.47) μg/L, P=0.466]. Furthermore, there was a significant decrease of modified Mayo score [-3(-4.75, -1.25) vs.-2(-3.25, 0), P=0.034] and a higher clinical remission rate (55.6% vs. 28.9%, P=0.020) at month 12 in study group than those in control group. In addition, according to the baseline level of serum 25(OH)D before mesalazine treatment, 74 UC patients were divided into vitamin D deficiency group ( n=38, serum 25(OH)D<20 μg/L) and non-deficiency group ( n=36, serum 25(OH)D≥20 μg/L). At month 12 in vitamin D deficiency group, patients with vitamin D3 supplementation had a greater decline in modified Mayo score [-4(-5.75, -2) vs.-2(-4, 0), P=0.048] and a higher clinical remission rate (60.0% vs. 22.2%, P=0.019) compared with those without. Conclusions:In patients with mild-to-moderate active UC receiving mesalazine treatment, vitamin D3 supplementation may improve the clinical efficacy, especially in patients with vitamin D deficiency.