Background: Uterine leiomyomas (fibroids) are the most common benign pelvic tumors in women of reproductive age with an incidence ranging from 5.4 to 77, Also, there is a high prevalence of vit D deficiency in Iran and there are the numbers of in vivo and vitro biological studies on the relationship of vitamin D and uterine leiomyomas. The aim of this study was to evaluate the effect of vit D supplementation on the size of uterine leiomyoma in women with vit D deficiency. Methods: This double-blind prospective clinical trial was performed on 69 patients with uterine leiomyomas who had vit D deficiency. Group A (n=35) was treated with vit D 50,000 IU every 2 weeks for 10 weeks, while group B (n=34) received placebo with same color and shape. Finally, the leiomyoma size in both groups was compared (IRCT: 20160521027998N5). Results: After a 10-week intervention, 25-hydroxyvitamin D3 levels were significantly higher in group receiving vitamin D (36.08 vs 16.25 ng/ml). (P<0.001) Leiomyomas size in vit D group significantly decreased as compared to placebo group (52.58 vs 61.11 mm, respectively). Conclusion: Our results showed that administration of vit D3 may reduce the size of leiomyoma. It seems that vitamin D administration is the effective way to treat leiomyoma
